Cambridge allows an accessory dwelling unit by right on any lot where residences are permitted: Zoning Ordinance Section 4.22, rewritten by Ordinance 2025-1 on February 10, 2025, now says only that accessory apartments are permitted in all such districts, and the state ADU law (G.L. c. 40A s. 3; 760 CMR 71.00) guarantees one unit of up to 900 sq ft or half the main house with no special permit, owner-occupancy or parking. The old Board of Zoning Appeal special permit, the 900 sq ft / 35% cap and the 1,800 sq ft minimum house size are gone. In Residence C-1, which now covers every former single- and two-family neighborhood, a unit needs 5-ft side and rear yards, a 10-ft front yard and 30% open space; a detached building faces the 15-ft accessory-structure height in Section 4.21(k) unless ISD applies the 45-ft dwelling limit under the state rule. The only city charge is the building permit at $15 per $1,000 of construction cost (about $3,750 on a $250,000 unit); there are no impact fees and no parking minimums.
Zoning & Size Requirements [1][2][3][4][5][6]
Yes — Cambridge allows ADUs. There is no square-footage cap — size is governed by height, setbacks and lot coverage — under current Massachusetts and Cambridge zoning rules.
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| Allowed Zones | Every district where residences are allowed, incl. Residence C-1 (all former A-1, A-2, B and C single- and two-family areas since Feb 10, 2025), C-1A to C-3B and the Office and Business districts (Zoning Ordinance 4.22 as amended by Ord. 2025-1). State Protected Use ADU rights (G.L. c. 40A s. 3; 760 CMR 71.03) apply wherever single-family use is allowed by right. Exterior work in a historic or neighborhood conservation district needs Cambridge Historical Commission approval first. |
| Max Size (Detached) | No local size cap since Feb 10, 2025 (Ord. 2025-1 struck the 900 sq ft / 35% rule in old 4.22.1). To keep state Protected Use status the unit must be 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is less; a larger unit is simply an additional dwelling unit, which Cambridge also allows as of right with no FAR limit for residences (5.25.4(a)) and no lot-area-per-unit rule. |
| Max Size (Attached) | Same: no local cap; 900 sq ft or 50% of the principal dwelling to stay a state Protected Use ADU. A basement or attic conversion inside the house faces no Cambridge FAR test (residential floor area is exempt from FAR, 5.25.4(a)) but must meet building-code ceiling-height, egress and light requirements. |
| Max Size (JADU) | Not a category in Massachusetts or Cambridge; a unit with its own kitchen and bath is an accessory apartment (4.22) or, if larger, an ordinary second dwelling unit. |
| Max Height | Residence C-1: 4 stories / 45 ft for residential buildings (Table 5-1, 5.31); 6 stories / 74 ft on a lot of 5,000+ sq ft that meets inclusionary rules (5.40 fn. 2). An accessory building in Res C-1 and Office-1 is capped at 15 ft (4.21(k)); 760 CMR 71.03(3)(b)2 bars applying a stricter standard to a Protected Use ADU than to the principal dwelling, so confirm with ISD zoning staff before designing a two-story backyard unit. |
| Max Units | No local cap: the 'no more than one accessory apartment per lot' clause of old 4.22.1(4) was struck Feb 10, 2025, and Cambridge no longer limits the number of dwelling units on a residential lot. The state guarantees one Protected Use ADU per lot by right; further units are reviewed as ordinary additional dwelling units under the 2025 multifamily zoning. |
Setback Requirements [1][2][3][4][5][6]
Cambridge requires a rear setback of about 5 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 | 5 ft in Residence C-1 for residential buildings (Table 5-1, col. 5); an accessory building may not sit closer to any lot line than the district minimum (4.21(h)). No setback where the building abuts a neighbor's building across a party wall (5.40 fn. 4(a)). Eaves, bay windows and similar projections of up to 3.5 ft may extend into the yard (5.24.1(f)). |
| Side Setback | 5 ft in Residence C-1; one side yard may be reduced if the two opposite side yards add up to at least 10 ft (5.40 fn. 4(b)). Other residence districts (C-1A to C-3A) also use 5 ft per Table 5-1. |
| Front Setback | 10 ft in Residence C-1 (5 ft in C-2A, C-3 and C-3A), but a dwelling need not sit back farther than the average of the buildings on the two adjacent lots (5.40 fn. 3). Check your district on the Feb 10, 2025 zoning map. |
| Parking Required | None. Cambridge eliminated all minimum parking requirements citywide by Ordinance #2022-5 (ordained Oct 24, 2022), and the state rule (no ADU parking within 0.5 mile of a transit station, at most 1 space elsewhere, 760 CMR 71.03(2)(b)) cannot add any back. A new or widened curb cut still needs City Council approval before the building permit is issued. |
| Owner Occupancy | Not required: 760 CMR 71.03(2)(a) prohibits an owner-occupancy requirement for either unit, and Cambridge's 4.22 imposes none. The Article 2 definition (Ord. #1408, 2019) keeps the accessory apartment under the same ownership as the main house, i.e. it cannot be sold off separately. |
| Min Lot Size | None. Ordinance 2025-1 removed minimum lot area, lot width and lot-area-per-dwelling-unit rules for residences citywide, and 760 CMR 71.03(3)(b)2 bars a minimum lot size for a Protected Use ADU. Open space: 30% of the lot in Res C-1, at least half of it permeable (Table 5-1, 5.40 fn. 1). |
Permit Process & Fees [7][8][9][10][11]
An ADU permit in Cambridge goes through a standard permit review. Review time: not published. ISD posts no review target for building permits; the state ADU law makes the unit as-of-right so there is no hearing, but Historical Commission review in a historic or conservation district and a curb-cut vote add weeks. Permit fees: Building permit $15 per $1,000 of construction cost for buildings of 3 dwelling units or fewer ($20 per $1,000 otherwise), $50 minimum (ISD Building Fees table): about $3,750 on a $250,000 ADU, $5,250 on $350,000. Plan review included; certificate of occupancy $100; trade permits extra.
| Approval Process | Building permit only, filed through ISD's OpenGov portal (cambridgema.portal.opengov.com); no BZA hearing since Ord. 2025-1 (Feb 10, 2025) deleted the 4.22 special-permit process. Steps: 1) confirm district and yards on the 2025 zoning map; 2) Cambridge Historical Commission sign-off if the property is in a historic or conservation district, is a landmark, or a building 50+ years old is to be demolished; 3) City Council curb-cut approval if a new driveway is needed; 4) submit plans with the estimated-cost affidavit and a homeowner's or licensed construction supervisor's affidavit; 5) separate electrical, plumbing and gas permits; 6) inspections, then a certificate of occupancy ($100 for the first unit, $50 per additional unit). |
| Review Timeline | Not published. ISD posts no review target for building permits; the state ADU law makes the unit as-of-right so there is no hearing, but Historical Commission review in a historic or conservation district and a curb-cut vote add weeks. |
| Permit Fees | Building permit $15 per $1,000 of construction cost for buildings of 3 dwelling units or fewer ($20 per $1,000 otherwise), $50 minimum (ISD Building Fees table): about $3,750 on a $250,000 ADU, $5,250 on $350,000. Plan review included; certificate of occupancy $100; trade permits extra. |
| Impact Fees | None. Cambridge has no residential impact fee; its Incentive Zoning housing contribution ($36.36 per sq ft) applies only to non-residential projects over 30,000 sq ft, and inclusionary housing (11.203) only to larger residential projects. 760 CMR 71.03(3)(b)6 bars any ADU-only impact fee. Water and sewer connection charges from DPW and the Water Department may apply to a detached unit; confirm before design. |
| Plan Check | Included in the building permit fee per the ISD Building Fees table; there is no separate ADU plan-check queue. |
| Pre-Approved Plans | Not available. Cambridge publishes no ADU program page, pre-approved plan catalogue or ADU handbook (CDD and ISD sites checked 2026-09-20). |
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Construction Costs [12][13][14][15]
Building an ADU in Cambridge typically costs $220,000-$400,000 ($275-$450 per sq ft); a garage conversion runs $100,000-$250,000.
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| Average Build Cost | $220,000-$400,000 |
| Cost per Sq Ft | $275-$450 |
| Garage Conversion | $100,000-$250,000 |
| Prefab ADU (Installed) | $250,000-$480,000 |
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Rental Income Potential [16][17][18][19]
A 2-bedroom ADU in Cambridge rents for around $3,800-$4,300/mo, an estimated 9-14% gross rent yield.
| Avg Rent (1 Bedroom) | $3,200-$3,600/mo |
| Avg Rent (2 Bedroom) | $3,800-$4,300/mo |
| Short-Term Rental | Only as an owner-adjacent short-term rental: the whole unit, inside a building of 4 or fewer units that the operator owns entirely and lives in (Zoning Ordinance 4.60, Ord. |
| Est. Annual ROI | 9-14% gross rent yield |
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Recent Law Changes
What's Changed Recently
- 2026-09-20 re-verification: the entry previously cited only state sources; it now rests on Cambridge Ordinances 2025-1 and 2025-2, the ISD fee tables, Zoning Ordinance 4.60 and HUD FY2027 rents. The BZA special permit, the local 900 sq ft / 35% cap and the 1,800 sq ft minimum house size described in older guides no longer exist.
- Ordinance 2025-1 (Multifamily Housing Zoning Petition, Part One; ordained and effective Feb 10, 2025) rewrote Section 4.22 to a single sentence, 'Accessory Apartments shall be permitted in all zoning districts where Residences are permitted', striking the BZA special permit, the 900 sq ft / 35% cap, the 1,800 sq ft minimum, the Feb 1, 2019 existing-building test and the one-per-lot clause; it merged Residence A-1, A-2, B and C into Residence C-1 (4 stories / 45 ft, 10-ft front and 5-ft side and rear yards, no FAR or lot-area-per-unit for residences) and removed the last minimum-parking references.
- Massachusetts ADU law (Ch. 150 of 2024 s. 8; 760 CMR 71.00 effective Jan 31, 2025, law in force Feb 2, 2025): one Protected Use ADU of up to 900 sq ft or 50% of the main house by right; no owner-occupancy requirement, no ADU parking within 0.5 mile of transit, no minimum lot size, no ADU-only impact fees or studies; municipalities may restrict short-term rental of ADUs (760 CMR 71.03(3)(c)).
- Ordinance #2022-5 (ordained Oct 24, 2022) eliminated all minimum accessory parking requirements citywide.
- Short-term rentals: a June 23, 2025 City Manager proposal (CMA 2025 #176) to amend Section 4.60 (28-day threshold, bedroom-only operator-occupied rentals) was not on the CDD adopted-amendments list as of 2026-09-20; the Ord. #1397 (Aug 7, 2017) rules still apply.
- Cambridge's last ADU-specific ordinance was #1408 (May 6, 2019); no ADU-specific amendment has been adopted since, because the Feb 2025 citywide rezoning removed the local restrictions.
Tips for Building an ADU in Cambridge
- Stay at 900 sq ft or 50% of the main house's gross floor area to keep state Protected Use status; a bigger unit is still allowed in Cambridge as an ordinary additional dwelling unit (no FAR for residences, no lot-area-per-unit), but it loses the state's overrides of local rules.
- Detached backyard unit: keep 5 ft from side and rear lines and 10 ft from the street (or the average of the neighbors' setbacks), and design to the 15-ft accessory-building height in 4.21(k) unless ISD confirms in writing that the 45-ft dwelling limit applies under 760 CMR 71.03(3)(b)2.
- Open space: 30% of the lot must stay open in Residence C-1, half of it permeable, so a detached unit plus a large patio can fail on a 3,000 sq ft lot; check the site plan before hiring a designer.
- Fee math: $15 per $1,000 of construction cost, so $3,000 at $200,000 and $4,500 at $300,000, plus a $100 certificate of occupancy and separate electrical, plumbing and gas permits. No impact, linkage or inclusionary fees apply to one ADU.
- Historic areas: a property in a historic or neighborhood conservation district, a designated landmark, or any demolition of a structure over 50 years old goes to the Cambridge Historical Commission before ISD will issue the permit.
- No parking is required anywhere in the city; a new curb cut needs a City Council vote, so an interior, basement or rear-yard unit reached on foot avoids that step.
- Short-term rental: only an attached or interior unit in a building of 4 or fewer units that you own and live in can be registered ($500) as an owner-adjacent STR; a detached cottage cannot on the ordinance's wording, so plan on 30-day-plus leases.
- Rents: HUD FY2027 Small Area FMRs for Cambridge ZIPs run $3,180-$3,610 for a 1-bedroom (02140 lowest, 02138 highest) and $3,800-$4,310 for a 2-bedroom, the highest of any city on this site.
- Cost basis (2026-09-20 re-verification): avg build cost $220,000-$400,000 = a 600-900 sq ft detached, stick-built unit per Dwelly (Mar 2026: $275-$450 per sq ft; 800 sq ft $220,000-$360,000) and Horizon ADU (Jul 2026: detached 900 sq ft $250,000-$360,000) | cost per sq ft $275-$450 (Dwelly; Horizon $300-$400) | garage or basement conversion $100,000-$250,000 (Horizon) | prefab $250,000-$480,000 installed for 650-750 sq ft in Greater Boston (ADU Wizard, 2026 edition).
- Cost basis (2026-09-20 re-verification): short term allowed: Only as an owner-adjacent short-term rental: the whole unit, inside a building of 4 or fewer units that the operator owns entirely and lives in (Zoning Ordinance 4.60, Ord. #1397); on that wording a detached backyard ADU does not qualify. ISD registration $500, one STR per operator, stays under 30 days. 760 CMR 71.03(3)(c) lets Cambridge keep these limits.
ADU Builders in Cambridge
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Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Ordinance No. 2025-1, Multifamily Housing Zoning Petition Part One, ordained as amended Feb 10, 2025 (final publication) Government Data — Accessed 2026-09-20
- Ordinance No. 2025-2, Multifamily Housing Zoning Petition Part Two, ordained Feb 10, 2025 (final publication) Government Data — Accessed 2026-09-20
- Zoning for Multifamily Housing, Cambridge Community Development Department Government Data — Accessed 2026-09-20
- Adopted Zoning Amendments, Cambridge CDD (Ord. #1408 Accessory Apartments May 6, 2019; Ord. #2022-5 Accessory Parking Requirements Oct 24, 2022; Multifamily Housing Feb 10, 2025) Government Data — Accessed 2026-09-20
- Accessory Apartments amendment by substitution (Section 4.22 text ordained as Ordinance #1408, 2019) Government Data — Accessed 2026-09-20
- 760 CMR 71.03, Regulation of Protected Use ADUs in Single-family Residential Zoning Districts Government Data — Accessed 2026-09-19
- Building Fees, Cambridge Inspectional Services Department Government Data — Accessed 2026-09-20
- Building Permits: How to Apply (OpenGov portal), Cambridge ISD Government Data — Accessed 2026-09-20
- Building / Electric / Plumbing Permits FAQ, Cambridge ISD Government Data — Accessed 2026-09-20
- Board of Zoning Appeal Fee Schedule, Cambridge ISD Government Data — Accessed 2026-09-20
- Incentive Zoning Ordinance, Cambridge CDD Government Data — Accessed 2026-09-20
- Dwelly, How Much Does an ADU Cost in Massachusetts in 2026? (Mar 7, 2026) industry — Accessed 2026-09-20
- Horizon ADU, How Much Does a 900 Sq Ft ADU Cost in Massachusetts? (Jul 24, 2026) industry — Accessed 2026-09-20
- ADU Wizard, How Much Does an ADU Cost in Massachusetts in 2026? industry — Accessed 2026-09-20
- River City Boston, Cost to Build an ADU in Cambridge MA: A 2026 Homeowner's Pricing Guide (Aug 8, 2026) industry — Accessed 2026-09-20
- HUD FY2027 Small Area Fair Market Rent Schedule (Cambridge ZIPs 02138-02142) Government Data — Accessed 2026-09-20
- HUD FY2027 Fair Market Rents, Boston-Cambridge-Quincy, MA-NH HMFA Government Data — Accessed 2026-09-20
- Short-Term Rental Guide, Cambridge Inspectional Services Department Government Data — Accessed 2026-09-20
- CMA 2025 #176, proposed Short-Term Rental Ordinance amendments (June 23, 2025) Government Data — Accessed 2026-09-20
- Accessory Dwelling Units, Mass.gov (EOHLC) Government Data — Accessed 2026-06-28
- The Zoning Ordinance, Cambridge CDD (article index; full text on Municode) Government Data — Accessed 2026-09-20
Frequently Asked Questions
Can I build an ADU in Cambridge?
Yes. ADUs are allowed in Every district where residences are allowed, incl. Residence C-1 (all former A-1, A-2, B and C single- and two-family areas since Feb 10, 2025), C-1A to C-3B and the Office and Business districts (Zoning Ordinance 4.22 as amended by Ord. 2025-1). State Protected Use ADU rights (G.L. c. 40A s. 3; 760 CMR 71.03) apply wherever single-family use is allowed by right. Exterior work in a historic or neighborhood conservation district needs Cambridge Historical Commission approval first. in Cambridge. The maximum size for a detached ADU is No local size cap since Feb 10, 2025 (Ord. 2025-1 struck the 900 sq ft / 35% rule in old 4.22.1). To keep state Protected Use status the unit must be 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is less; a larger unit is simply an additional dwelling unit, which Cambridge also allows as of right with no FAR limit for residences (5.25.4(a)) and no lot-area-per-unit rule.. Owner-occupancy is not required, so you can rent out the ADU.
How much does an ADU cost in Cambridge?
ADU construction in Cambridge typically costs $220,000-$400,000. Garage conversions run $100,000-$250,000, and prefab ADUs cost $250,000-$480,000.
How much can I rent an ADU for in Cambridge?
Average ADU rents in Cambridge are $3,200-$3,600/mo for a 1-bedroom and $3,800-$4,300/mo for a 2-bedroom unit.
Do I need parking for an ADU in Cambridge?
None. Cambridge eliminated all minimum parking requirements citywide by Ordinance #2022-5 (ordained Oct 24, 2022), and the state rule (no ADU parking within 0.5 mile of a transit station, at most 1 space elsewhere, 760 CMR 71.03(2)(b)) cannot add any back. A new or widened curb cut still needs City Council approval before the building permit is issued.. Check with Cambridge 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 Cambridge?
Cambridge does not publish a standard ADU review timeline. ISD posts no review target for building permits; the state ADU law makes the unit as-of-right so there is no hearing, but Historical Commission review in a historic or conservation district and a curb-cut vote add weeks. Permit fees: Building permit $15 per $1,000 of construction cost for buildings of 3 dwelling units or fewer ($20 per $1,000 otherwise), $50 minimum (ISD Building Fees table): about $3,750 on a $250,000 ADU, $5,250 on $350,000. Plan review included; certificate of occupancy $100; trade permits extra.
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