Waltham is one of the Massachusetts cities that actually did the work. Ordinance No. 36552, passed on its third and final reading on April 28, 2025, struck the city's old accessory dwelling unit rules in their entirety and replaced them: a single ADU is now permitted by right, the Board of Appeals special permit is gone, the owner-occupancy requirement is gone, and the old minimum lot sizes of 20,000 sq ft in Residence A-1 and 15,000 in A-2 are gone with them. The Building Department publishes a proper ADU application package with a checklist, and the parking rule tracks 760 CMR 71.00 exactly — one off-street space, or none at all if the lot is within half a mile of a transit station, which the city will let you prove with a radius map from its own GIS. Three things to watch. First, Waltham imposes a 450 sq ft minimum floor area that state law does not, so a small basement conversion may not qualify. Second, § 3.23(c) tells you to meet the underlying district's dimensional requirements, which in RA-1 means a 40-foot front, 20-foot side and 40-foot rear setback — but 760 CMR 71.03(3)(b)(2) entitles a protected-use ADU to whichever of the principal-dwelling or accessory-structure standard is more permissive, and Waltham's own table gives an accessory building 5 feet at the side and rear. Third, the Building Department's website still links a Table of Uses scan current only through 2017, which shows ADUs needing a Board of Appeals special permit in four districts. That document is two years out of date; the ordinance is the one that governs.
Zoning & Size Requirements [1][2][3][4][5][6]
Yes — Waltham allows ADUs. A detached ADU can typically be up to 900 sq ft under current Massachusetts and Waltham zoning rules.
See what this means for your lot — get 5 free quotes from Waltham ADU builders →
| Allowed Zones | Permitted (Y) in RA-1, RA-2, RA-3, RA4, RB, RC, RD, HR1, BA and BB; not permitted (N) in HR2, BC, LC, C, I and CR — the Table of Uses row for Accessory Dwelling Units as replaced by Ordinance No. 36552 (third and final reading April 28, 2025), with a use reference to §§ 3.23 and 3.616. The HR2 "N" looks preempted: single-family detached is itself a permitted use in HR2 under § 3.4, and 760 CMR 71.02 defines a "Single-family Residential Zoning District" as "[a]ny Zoning District where Single-family Residential Dwellings are a permitted or an allowable use," so M.G.L. c. 40A § 3 should protect one ADU there too. If your lot is in HR2, raise the statute with the Building Department at 781-314-3275 before you accept a refusal. Note also that the Table of Uses scan the city links from its Building page is current only through Ord. No. 33817 of August 1, 2017 (Supplement No. 15) and still shows the repealed row — see recent_changes |
| Max Size (Detached) | Between 450 and 900 sq ft. "The minimum floor area of an ADU shall not be less than 450-square feet. The maximum floor area of an ADU shall not exceed 900-square feet, or 50% of the total gross floor area of the principal dwelling unit, whichever is smaller" (Waltham Zoning Code § 3.23(b), Ord. No. 36552). The upper limit matches the state definition at M.G.L. c. 40A § 1A exactly, and the practical effect is the same everywhere in Massachusetts: on a house under 1,800 sq ft of gross floor area the half rule binds before the 900 does. Gross floor area is measured under 760 CMR 71.02 — all stories of compliant ceiling height including basements and lofts, excluding crawl spaces, garage parking areas, attics and enclosed porches — and Waltham's own ADU application form says so in terms. The 450 sq ft floor is Waltham's own addition with no counterpart in the statute or the regulation; whether it survives 760 CMR 71.03(3)(a)(3), which makes a restriction unreasonable if it nullifies the use or imposes excessive cost without advancing a municipal interest, has not been tested. If your conversion comes in under 450 sq ft, ask the Building Department how it is being applied before you redraw |
| Max Size (Attached) | The same: not less than 450 sq ft and not more than the smaller of 900 sq ft or 50 percent of the principal dwelling's total gross floor area (Waltham Zoning Code § 3.23(b)). Waltham's ADU application form recognises four types — attached, detached, internal and secondary — and applies the same size rule to all of them; a secondary unit is the one that needs a special permit under § 3.616 |
| Max Size (JADU) | Not a category in Massachusetts. Neither M.G.L. c. 40A, nor 760 CMR 71.00, nor the Waltham Zoning Code recognises a junior ADU. An interior unit carved out of the house is an "internal" ADU on Waltham's application form and takes the ordinary 450-900 sq ft rule |
| Max Height | The maximum building height of the underlying district — 35 feet and 2½ stories in RA-1, RA-2, RA-3 and RA-4, 40 feet and 3 stories in RB and RC, 50 feet and 4 stories in RD, and 74 feet and 6 stories in HR1 (Waltham Zoning Code § 4.11, Table of Dimensional Regulations). Section 3.23(c) is explicit that an ADU "must comply with the dimensional requirements for the underlying zoning district, including, but not limited to, frontage, maximum height, minimum setback, and maximum lot coverage requirements." Waltham's separate accessory-building rules are tighter — one story, and above 15 feet the height may not exceed 125 percent of the least horizontal dimension or 20 feet in any case (§ 4.223, and the accessory-building row of Table 4.11) — but 760 CMR 71.03(3)(b)(2) entitles a protected-use ADU to the more permissive of the two, which is the district standard. Expect to have to make that argument for a two-storey detached unit |
| Max Units | 1 by right; a second needs a City Council special permit. "A single ADU, accessory to a principal dwelling unit, is permitted by-right. The locating of more than one ADU on a single lot shall require a special permit from the City Council, consistent with Section 3.616" (Waltham Zoning Code § 3.23(i)). That matches 760 CMR 71.03(5), which requires a special permit for additional ADUs beyond the protected one. The § 3.616 criteria as rewritten by Ordinance No. 36552 are that the secondary ADU meets all of § 3.23, that its design and exterior appearance are compatible with the principal dwelling and the neighbourhood, that a sprinkler system is installed wherever four or more dwelling units including the principal are in one structure, and that each additional ADU has a dedicated off-street space unless the lot is within half a mile of a transit station. The protected ADU itself is not counted in density: 760 CMR 71.03(2)(d) provides that "Protected Use ADUs shall not be counted in any density calculations," which matters in Waltham because Table 4.11 sets maximum dwelling units per acre in several districts |
Setback Requirements [1][2][3][4][5][6]
Waltham requires a rear setback of about 40 ft for a detached ADU, and one off-street parking space is required. Owner-occupancy is not required — you can rent it out without living on the property.
| Rear Setback | Read the ordinance and you get the district's rear yard: 40 feet in RA-1 and RA-2, 30 feet in RA-3, RA-4 and RB, 20 feet in RC and HR1, 60 feet in RD (Waltham Zoning Code § 4.11). Read the state regulation and you get 5 feet. Waltham's own standard for an accessory building is "at least 10 feet from the principal building and five feet from the side lot line and the rear lot line" (§ 4.221, and the accessory-building row of Table 4.11), and 760 CMR 71.03(3)(b)(2) makes unreasonable any dimensional standard more restrictive than what applies to "the Principal Dwelling, or a Single-family Residential Dwelling or accessory structure ... whichever results in more permissive regulation." On a 40-foot RA-1 rear yard that is the difference between a buildable back garden and none. Two local rules still bite: the 10-foot separation from the house, and § 4.221's limit that a detached accessory building may occupy "not more than 25% of the required rear yard area." Confirm the approach with the Building Department at 781-314-3275 before you site the foundation |
| Side Setback | The district side yard under the ordinance — 20 feet in RA-1 and RA-2, 15 feet in RA-3 and RA-4, 10 feet in RB, RC and HR1, 45 feet in RD (Waltham Zoning Code § 4.11) — against 5 feet for an accessory building under § 4.221 and Table 4.11. As with the rear yard, 760 CMR 71.03(3)(b)(2) entitles a protected-use ADU to the more permissive of the two. Footnote (3) to Table 4.11 adds that in RC the side yard is 10 feet where the building does not exceed 2½ stories, and otherwise a quarter of the building's height |
| Front Setback | The district front yard, with no accessory-structure relief available: 40 feet in RA-1 and RA-2, 25 feet in RA-3 and RA-4, 15 feet in RB, 10 feet in RC and HR1, 75 feet in RD (Waltham Zoning Code § 4.11). The accessory-building row of Table 4.11 gives the front setback as "As required by district," so unlike the side and rear yards there is nothing more permissive to claim. Plan a detached ADU for the rear of the lot |
| Parking Required | One off-street space — unless you are near transit, and most of Waltham is. "Each ADU shall provide at least one off-street parking space in addition to the parking required for the principal dwelling unit, unless the lot on which the ADU is located is within 0.5 miles from a Transit Station, as defined by 760 CMR 71.00" (Waltham Zoning Code § 3.23(d)). The city has built the exemption into its process: the ADU application form has a tick box for "ADU Lot is located within 0.5 miles / 2,640ft from a Transit Station" and asks you to attach a radius map from Waltham's own MapGeo GIS to prove it. Waltham has Fitchburg Line commuter rail at Waltham (Central Square) and Brandeis/Roberts, and 760 CMR 71.02 defines a "Bus Station" as "[a] location serving as a point of embarkation for any bus operated by a Transit Authority" — which on its face takes in the MBTA bus stops on routes serving the city. Either way, one space is the ceiling: 760 CMR 71.03(2)(b)(1) forbids requiring more than one, even for a lot entirely outside the half-mile radius |
| Owner Occupancy | Not required, and not merely superseded — Waltham repealed it. Ordinance No. 36552 struck the former § 3.23 "in its entirety," and the replacement contains no owner-occupancy condition, no recorded covenant, no re-certification and no family-relationship rule. The old regime, still visible in the city's archived Zoning Board of Appeals agendas as "Application for Special Permit – Accessory Dwelling Unit" and "re-certification of Accessory Dwelling Unit," is gone. State law would bar it in any event: M.G.L. c. 40A § 3 provides that an ADU "shall not require owner occupancy of either the accessory dwelling unit or the principal dwelling," and 760 CMR 71.03(2)(a) lists it as a Prohibited Regulation. Under 760 CMR 71.03(4) the city may not enforce such a condition even where it was attached to an ADU approved before January 31, 2025 — worth knowing if you hold an old Waltham ADU special permit with an owner-occupancy condition on it |
| Min Lot Size | None may be required for the ADU: "a Municipality may not require a minimum Lot size for a Protected Use ADU" (760 CMR 71.03(3)(b)(2)). This is a real change in Waltham, because the repealed § 3.23 conditioned an accessory unit on a lot of at least 20,000 sq ft in Residence A-1 and 15,000 sq ft in Residence A-2. The district minimums for the lot itself remain: 20,000 sq ft with 100 feet of frontage in RA-1, 15,000 sq ft with 80 feet in RA-2, 9,600 sq ft with 70 feet in RA-3, 7,000 sq ft with 60 feet in RA-4, 6,000 sq ft with 60 feet for a single-family dwelling in RB, 6,000 sq ft with 50 feet in RC, 5 acres in RD and 10,000 sq ft with 50 feet in HR1 (Waltham Zoning Code § 4.11). Maximum lot coverage is 20 percent in RA-1 and RA-2, 25 percent in RA-3 and RA-4, and 30 percent in RB, RC and RD — and footnote (2) to Table 4.11 counts "all principal and accessory buildings on any lot," so a detached ADU eats into it |
Permit Process & Fees [7][8][9][10]
An ADU permit in Waltham goes through a standard permit review. Confirm with the Waltham Building Department at 781-314-3275 — the city publishes no ADU-specific review timeline. What has changed since April 2025 is the shape of the process rather than its speed: an ADU no longer goes to the Zoning Board of Appeals, so there is no hearing date, no abutter notice period on a special permit and no appeal window to clear. Budget instead for the surveyor's as-built and the fire department plan stamp, which are on the checklist and are the usual cause of delay. Permit fees: Residential building permit fees are $12 per $1,000 of the estimated cost of the job, with a $50 minimum — the city computes them as total cost of construction × 0.012 (City of Waltham Building Department). On a $250,000 ADU that is about $3,000. Add the separate trades: residential plumbing at $25 for the first fixture and $10 for each additional, residential gas from $30, and electrical on the Wires Department schedule. If a detached unit needs a utility trench in the public way, the Trench Permit Fee Schedule charges $50 for a single residential trench and $100 maximum for a residential multi-trench. Commercial rates of $22 per $1,000 do not apply to an ADU.
| Approval Process | A building permit — no special permit, no hearing, no covenant for a single ADU. "A building permit is required for the construction or conversion of an ADU. The building permit application shall include detailed plans that demonstrate compliance with this chapter, including dimensional compliance, parking requirements, and the total square footage of the ADU" (Waltham Zoning Code § 3.23(h)). The Building Department publishes an ADU Application Package with a property-information form and a checklist: total gross floor area of the existing basement and floor plans for the entire structure; an as-built from a professional land surveyor for an attached or detached unit; zoning district and ward; unit size; parking compliance with 760 CMR 71.00; water and sewer per the Engineering guidelines; and dimensional compliance with the underlying district. The general building-permit checklist adds a workers' compensation affidavit and certificate of insurance, copies of the CSL and HIC (which must be held by the same person), a waste debris form, a certified letter to abutters with the green certified forms for a new structure or addition, fire department stamped plans showing the smoke and CO detector layout, a certified plot plan, a HERS rating, a sheet metal form, a habitable space form if you are finishing a basement, an Engineering I&I sign-off, and the entire submittal package on a thumb drive. Applications filed after July 1, 2025 must comply with the 10th Edition of the Massachusetts State Building Code. Building Department, Government Center, 119 School Street, Room 25: 781-314-3275; ask for Sandra Vidal at 781-314-3284 for a walk-through of the process. Planning Department, 25 Lexington Street, 2nd Floor: 781-314-3370 |
| Review Timeline | Confirm with the Waltham Building Department at 781-314-3275 — the city publishes no ADU-specific review timeline. What has changed since April 2025 is the shape of the process rather than its speed: an ADU no longer goes to the Zoning Board of Appeals, so there is no hearing date, no abutter notice period on a special permit and no appeal window to clear. Budget instead for the surveyor's as-built and the fire department plan stamp, which are on the checklist and are the usual cause of delay |
| Permit Fees | Residential building permit fees are $12 per $1,000 of the estimated cost of the job, with a $50 minimum — the city computes them as total cost of construction × 0.012 (City of Waltham Building Department). On a $250,000 ADU that is about $3,000. Add the separate trades: residential plumbing at $25 for the first fixture and $10 for each additional, residential gas from $30, and electrical on the Wires Department schedule. If a detached unit needs a utility trench in the public way, the Trench Permit Fee Schedule charges $50 for a single residential trench and $100 maximum for a residential multi-trench. Commercial rates of $22 per $1,000 do not apply to an ADU |
| Impact Fees | None. Massachusetts municipalities have no general ADU impact fee power, and 760 CMR 71.03(3)(b)(6) makes unreasonable "[a]ny requirement for any impact analysis, study, report, or impact fee that is not required for the development of a Single-family Residential Dwelling" in the same district. Waltham's ADU ordinance imposes none |
| Plan Check | Standard building plan review against the 10th Edition of the Massachusetts State Building Code, plus a zoning check against § 3.23 and the district's dimensional requirements. Two items specific to Massachusetts. Utilities: § 3.23(f) requires the ADU to be "connected to the public water and sewer system or comply with applicable Title V requirements pursuant to 310 CMR 15.000," and the city's checklist routes water and sewer for detached, attached and basement units through the Engineering guidelines — but note that 760 CMR 71.03(3)(b)(3) says a municipality "may not require a separate utility connection, such as water, sewer, electric," though the utility itself, a state or regional board, or a court still can. Egress: § 3.23(g) requires a separate entrance, directly from outside or through a shared entry hall or corridor, sufficient for safe egress under the state building code — the same wording as the statutory definition at M.G.L. c. 40A § 1A. Every ADU must also be assigned an address to the MassGIS standard and reported to MassGIS and EOHLC (760 CMR 71.03(8)) |
| Pre-Approved Plans | None from the City of Waltham — the Building Department's ADU package assumes a custom set of plans and a surveyor's as-built, and no city plan gallery appears on its pages as of September 1, 2026. The Commonwealth fills the gap: EOHLC's Accessory Dwelling Units page hosts a free, searchable library of downloadable ADU designs, along with financing and technical-assistance programmes. Confirm with the Building Department at 781-314-3275 that a state design can be adapted to your lot before you buy anything |
Get Free ADU Quotes in Waltham
Tell us about your project and we'll connect you with ADU builders serving Waltham. Free, no obligation — you choose who to work with.
1. Tell us your project
Takes about a minute.
Takes about a minute.
2. We match local pros
Up to 5 independent local ADU builders — never more, and we tell them nothing you don't submit here.
Up to 5 independent local ADU builders — never more, and we tell them nothing you don't submit here.
3. You choose
Compare quotes, verify license & insurance, hire (or don't). No spam either way.
Compare quotes, verify license & insurance, hire (or don't). No spam either way.
🔒 Your info is secure. We share it only with contractors quoting your project. No spam, no obligation.
Just browsing? Search ADU builders near you by ZIP code.
Construction Costs [11]
Building an ADU in Waltham typically costs $235,000-$420,000 ($300-$500 per sq ft); a garage conversion runs $115,000-$235,000.
See what this means for your lot — get 5 free quotes from Waltham ADU builders →
| Average Build Cost | $235,000-$420,000 |
| Cost per Sq Ft | $300-$500 |
| Garage Conversion | $115,000-$235,000 |
| Prefab ADU (Installed) | $170,000-$340,000 |
Finance Your ADU Project
Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders.
Some links on this page are affiliate links. We may earn a commission at no extra cost to you. Learn more
Rental Income Potential [12][13]
A 2-bedroom ADU in Waltham rents for around $2,700-$3,400/mo, an estimated 5-8% annual return.
| Avg Rent (1 Bedroom) | $2,200-$2,700/mo |
| Avg Rent (2 Bedroom) | $2,700-$3,400/mo |
| Short-Term Rental | No. "The ADU shall be used exclusively for residential purposes and shall be prohibited for customary home occupation uses or short-term rentals in which at least one or more rooms, or the entire dwelling unit is rented to an occupant or sub-occupant for a period less than 31 consecutive days" (Waltham Zoning Code § 3.23(e)). The short-term rental half of that is expressly permitted by state law — M.G.L. c. 40A § 3 allows an ADU to "be subject to restrictions and prohibitions on short-term rental, as defined in section 1 of chapter 64G," and 760 CMR 71.03(3)(c) repeats it. The ban on customary home occupations is Waltham's own addition and sits awkwardly beside § 3.4, which permits customary home occupations as of right throughout the residential districts; if you were planning to work from the ADU, ask the Building Department how it reads that. Long-term letting of the unit is unrestricted, and with owner occupancy repealed you may rent both units |
| Est. Annual ROI | 5-8% annual return |
Explore Prefab ADU Options
Prefab ADUs can save 20-40% vs. traditional construction and cut build time in half.
Some links on this page are affiliate links. We may earn a commission at no extra cost to you. Learn more
Recent Law Changes
What's Changed Recently
- The Affordable Homes Act, St. 2024, c. 150 §§ 7-8, effective February 2, 2025, amended M.G.L. c. 40A §§ 1A and 3 so that no zoning ordinance may "prohibit, unreasonably restrict or require a special permit or other discretionary zoning approval for the use of land or structures for a single accessory dwelling unit, or the rental thereof, in a single-family residential zoning district." Owner occupancy may not be required; not more than one parking space may be required, and none within half a mile of a commuter rail station, subway station, ferry terminal or bus station
- EOHLC promulgated 760 CMR 71.00, Protected Use Accessory Dwelling Units, effective January 31, 2025 (Mass. Register Issue 1540), setting out the Prohibited Regulations and the test for an Unreasonable Regulation, and providing at 760 CMR 71.03(4) that a municipality may not enforce such conditions even where imposed before that date
- Waltham Ordinance No. 36552 — first reading March 24, 2025, second reading April 14, 2025, third and final reading April 28, 2025 — amended Article II § 2.302, Article III § 3.23, the § 3.4 Table of Uses and § 3.616 of the Zoning Code. It struck the former § 3.23 in its entirety, replaced the Table of Uses row so that ADUs are permitted by right rather than by Board of Appeals special permit, and rewrote § 3.616 to govern only special permits for more than one ADU on a lot
- What the ordinance repealed is as important as what it enacted: the Board of Appeals special permit, the owner-occupancy requirement, the five-yearly re-certification, and the minimum lot sizes of 20,000 sq ft in Residence A-1 and 15,000 sq ft in Residence A-2 that used to gate an accessory unit
- Waltham added a 450 sq ft minimum floor area for an ADU, which has no counterpart in M.G.L. c. 40A or 760 CMR 71.00. It is the one place where Waltham's rules are stricter than the Commonwealth's, and its enforceability against a smaller conversion is untested
- Stale mirror on the city's own site: the Building Department page links a "Table of Uses" scan of Zoning Code § 3.4 that is current only through Ordinance No. 33817 of August 1, 2017 (Supplement No. 15). It still shows Accessory dwelling units as "S2 — Use permitted only by special permit by Board of Appeals" in RA-1, RA-2, RA-3 and RA-4 and "N" everywhere else. That row was struck by Ordinance No. 36552 on April 28, 2025. The controlling text is the ordinance, which the same page links as "Accessory Dwelling Unit Regulations"
- The Table of Uses scan does still supply one useful fact the ordinance does not: single-family detached dwellings are a permitted use (Y) in RA-1 through RD, HR1 and HR2, and allowed by City Council special permit (S1) in BA and BB. Under the 760 CMR 71.02 definition that makes all of those "Single-family Residential Zoning Districts," which is why the ordinance's "N" for HR2 looks preempted
- The Waltham Zoning Code is hosted on eCode360, which blocks automated access; the text verified for this entry came from the City of Waltham's own PDFs of Ordinance No. 36552, the ADU Application Package, the § 4.11 Table of Dimensional Regulations and the § 4.12 footnotes. Confirm nothing has changed with the Building Department at 781-314-3275 before you rely on a figure here
Tips for Building an ADU in Waltham
- Ignore the Table of Uses PDF on the Building Department page. It is a 2017 scan and still shows an ADU needing a Board of Appeals special permit. The document that governs is Ordinance No. 36552, linked from the same page as "Accessory Dwelling Unit Regulations"
- The 450 sq ft floor is the trap nobody warns you about. Waltham will not permit an ADU smaller than 450 sq ft, which rules out the compact basement or above-garage conversions that work elsewhere in Massachusetts. Design to 450 or more, or ask the Building Department how it applies the rule before you spend on drawings
- Fight for the accessory-structure setbacks. Section 3.23(c) points you at the district's dimensional table — 40 feet at the rear in RA-1 — but 760 CMR 71.03(3)(b)(2) entitles you to whichever standard is more permissive, and Waltham's own accessory-building rule is 5 feet at the side and rear. On a standard RA-1 lot that is the whole project
- Get the radius map early. The city's ADU form will waive the parking space outright if you attach a map from Waltham's MapGeo GIS showing the lot within 2,640 feet of a transit station — and 760 CMR 71.02 counts an MBTA bus stop as a Transit Station, not just the Fitchburg Line platforms at Central Square and Brandeis/Roberts
- If you hold an old Waltham ADU special permit with an owner-occupancy or re-certification condition on it, 760 CMR 71.03(4) says the city may not enforce it — it applies to conditions imposed before January 31, 2025 as well as after
- Line up the surveyor before anything else. The checklist requires an as-built from a professional land surveyor for any attached or detached unit, plus a certified plot plan for a new structure, and those are the long-lead items
- Budget for the trades separately. The $12 per $1,000 building permit fee is only part of it; plumbing, gas, electrical and, for a detached unit needing a utility trench in the street, a trench permit all come on their own schedules
- In HR2 the ordinance says no, but single-family detached is a permitted use there — which makes HR2 a Single-family Residential Zoning District under 760 CMR 71.02 and brings the M.G.L. c. 40A § 3 protection with it. Put the argument in writing to the Inspector of Buildings rather than abandoning the project
ADU Builders in Waltham
Looking for an ADU builder in Waltham? ADU projects need a contractor experienced with local zoning, the permit process, and utility connections — not every general contractor has built one. We match Waltham homeowners with experienced local ADU contractors who provide free, no-obligation quotes. Getting at least three bids typically saves 20-40% on the same scope.
Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Waltham Ordinance No. 36552 — an ordinance amending Art. II § 2.302, Art. III § 3.23, § 3.4 Table of Uses and § 3.616 of the Zoning Code to regulate accessory dwelling units (third and final reading April 28, 2025) Municipal Code — Accessed 2026-09-01
- Waltham Zoning Code § 4.11, Table of Dimensional Regulations (Z Attachment 5, Supp. No. 14) Municipal Code — Accessed 2026-09-01
- Waltham Zoning Code § 4.12, Footnotes to the Table of Dimensional Requirements, and Art. IV § 4.22 accessory building provisions Municipal Code — Accessed 2026-09-01
- Waltham Zoning Code § 3.4, Table of Uses (Z Attachment 4, Supp. No. 15 — current only through Ord. No. 33817 of 8-1-2017; the ADU row is superseded by Ord. No. 36552) Municipal Code — Accessed 2026-09-01
- ADU Law — Acts of 2024, Chapter 150, Sections 7 and 8 (full text of the amendments to M.G.L. c. 40A §§ 1A and 3), EOHLC state_statute — Accessed 2026-09-01
- 760 CMR 71.00: Protected Use Accessory Dwelling Units — EOHLC, adopted Mass. Register Issue 1540, eff. 1/31/2025 state_statute — Accessed 2026-09-01
- City of Waltham ADU Application Package — ADU application form, ADU check list, Ordinance No. 36552 and Zoning Code § 4.22 accessory building provisions (Building Department) Planning Department — Accessed 2026-09-01
- Building Department — City of Waltham (permit requirements, residential fee of $12 per $1,000, 10th Edition code, contacts) Planning Department — Accessed 2026-09-01
- Trench Permit Fee Schedule — City of Waltham Building Department Planning Department — Accessed 2026-09-01
- Accessory Dwelling Units — EOHLC (FAQ, free ADU design library, Title 5 and MassGIS guidance) Government Data — Accessed 2026-09-01
- Angi — ADU Construction Cost Data Market Data — Accessed 2026-09-01
- Zillow Rental Manager — Market Rent Data Market Data — Accessed 2026-09-01
- HUD Fair Market Rents Government Data — Accessed 2026-09-01
- Planning Department — City of Waltham, 25 Lexington Street 2nd Floor, 781-314-3370 Planning Department — Accessed 2026-09-01
- U.S. Census Bureau — 2020 Decennial Census, Waltham city, Massachusetts (65,218) Government Data — Accessed 2026-09-01
Frequently Asked Questions
Can I build an ADU in Waltham?
Yes. ADUs are allowed in Permitted (Y) in RA-1, RA-2, RA-3, RA4, RB, RC, RD, HR1, BA and BB; not permitted (N) in HR2, BC, LC, C, I and CR — the Table of Uses row for Accessory Dwelling Units as replaced by Ordinance No. 36552 (third and final reading April 28, 2025), with a use reference to §§ 3.23 and 3.616. The HR2 "N" looks preempted: single-family detached is itself a permitted use in HR2 under § 3.4, and 760 CMR 71.02 defines a "Single-family Residential Zoning District" as "[a]ny Zoning District where Single-family Residential Dwellings are a permitted or an allowable use," so M.G.L. c. 40A § 3 should protect one ADU there too. If your lot is in HR2, raise the statute with the Building Department at 781-314-3275 before you accept a refusal. Note also that the Table of Uses scan the city links from its Building page is current only through Ord. No. 33817 of August 1, 2017 (Supplement No. 15) and still shows the repealed row — see recent_changes in Waltham. The maximum size for a detached ADU is Between 450 and 900 sq ft. "The minimum floor area of an ADU shall not be less than 450-square feet. The maximum floor area of an ADU shall not exceed 900-square feet, or 50% of the total gross floor area of the principal dwelling unit, whichever is smaller" (Waltham Zoning Code § 3.23(b), Ord. No. 36552). The upper limit matches the state definition at M.G.L. c. 40A § 1A exactly, and the practical effect is the same everywhere in Massachusetts: on a house under 1,800 sq ft of gross floor area the half rule binds before the 900 does. Gross floor area is measured under 760 CMR 71.02 — all stories of compliant ceiling height including basements and lofts, excluding crawl spaces, garage parking areas, attics and enclosed porches — and Waltham's own ADU application form says so in terms. The 450 sq ft floor is Waltham's own addition with no counterpart in the statute or the regulation; whether it survives 760 CMR 71.03(3)(a)(3), which makes a restriction unreasonable if it nullifies the use or imposes excessive cost without advancing a municipal interest, has not been tested. If your conversion comes in under 450 sq ft, ask the Building Department how it is being applied before you redraw. Owner-occupancy is not required, so you can rent out the ADU.
How much does an ADU cost in Waltham?
ADU construction in Waltham typically costs $235,000-$420,000. Garage conversions run $115,000-$235,000, and prefab ADUs cost $170,000-$340,000.
How much can I rent an ADU for in Waltham?
Average ADU rents in Waltham are $2,200-$2,700/mo for a 1-bedroom and $2,700-$3,400/mo for a 2-bedroom unit.
Do I need parking for an ADU in Waltham?
One off-street space — unless you are near transit, and most of Waltham is. "Each ADU shall provide at least one off-street parking space in addition to the parking required for the principal dwelling unit, unless the lot on which the ADU is located is within 0.5 miles from a Transit Station, as defined by 760 CMR 71.00" (Waltham Zoning Code § 3.23(d)). The city has built the exemption into its process: the ADU application form has a tick box for "ADU Lot is located within 0.5 miles / 2,640ft from a Transit Station" and asks you to attach a radius map from Waltham's own MapGeo GIS to prove it. Waltham has Fitchburg Line commuter rail at Waltham (Central Square) and Brandeis/Roberts, and 760 CMR 71.02 defines a "Bus Station" as "[a] location serving as a point of embarkation for any bus operated by a Transit Authority" — which on its face takes in the MBTA bus stops on routes serving the city. Either way, one space is the ceiling: 760 CMR 71.03(2)(b)(1) forbids requiring more than one, even for a lot entirely outside the half-mile radius. Check with Waltham planning department for the latest Massachusetts ADU parking rules, as many states have relaxed requirements in recent years.
How long does it take to get an ADU permit in Waltham?
Waltham does not publish a standard ADU review timeline — confirm with the Waltham Building Department at 781-314-3275 — the city publishes no ADU-specific review timeline. What has changed since April 2025 is the shape of the process rather than its speed: an ADU no longer goes to the Zoning Board of Appeals, so there is no hearing date, no abutter notice period on a special permit and no appeal window to clear. Budget instead for the surveyor's as-built and the fire department plan stamp, which are on the checklist and are the usual cause of delay. Permit fees: Residential building permit fees are $12 per $1,000 of the estimated cost of the job, with a $50 minimum — the city computes them as total cost of construction × 0.012 (City of Waltham Building Department). On a $250,000 ADU that is about $3,000. Add the separate trades: residential plumbing at $25 for the first fixture and $10 for each additional, residential gas from $30, and electrical on the Wires Department schedule. If a detached unit needs a utility trench in the public way, the Trench Permit Fee Schedule charges $50 for a single residential trench and $100 maximum for a residential multi-trench. Commercial rates of $22 per $1,000 do not apply to an ADU.
Ready to Build in Waltham?
Get free, no-obligation quotes from top-rated ADU contractors in Waltham.
Get Free Quotes →Build ADUs in Waltham?
Homeowners read this page while planning, then ask us to connect them with a local builder. We pass those requests to builders who cover Waltham — a flat fee only on the leads you accept, no subscription, and you contract directly with the homeowner.
See who's waiting in Massachusetts →