Boston allows an ADU by right in only one form: an interior ADU carved out of the existing footprint of an owner-occupied 1-, 2- or 3-family home (citywide Article 2 text amendment, BPDA Board April 11, 2019), which needs an Inspectional Services long-form permit ($50 plus $10 per $1,000 of construction cost) and no zoning relief. A detached or attached ADU is by right only in Mattapan's R1 and R2 subdistricts (Article 60, Zoning Commission January 2024): a detached unit up to a 900 sq ft floor plate and 1.5 stories, 5 ft from other buildings, with 5 ft front, 3 ft side and 20 ft rear yards. Everywhere else a detached or attached ADU needs a Zoning Board of Appeal variance ($150). The Neighborhood Housing rezoning that would allow ADUs without a variance was released as a draft for Hyde Park, Roslindale and West Roxbury on July 17, 2026 and is not adopted. Boston zones under its own 1956 enabling act rather than G.L. c. 40A, so the statewide by-right ADU law in force since February 2, 2025 does not apply in the city.
Zoning & Size Requirements [1][2][3][4][5][6]
Sometimes — Boston allows ADUs, but not by right on a typical lot. An interior ADU (a basement, attic or first-floor unit built entirely within the existing footprint of an owner-occupied 1-, 2- or 3-family home) is by right citywide and needs only an Inspectional Services long-form building permit. A detached or attached ADU is by right only in Mattapan's R1 and R2 subdistricts (Article 60, adopted January 2024); everywhere else it needs a Zoning Board of Appeal variance ($150 filing fee), because the code has no rule that allows it.
| Allowed Zones | Interior ADU by right citywide in any owner-occupied 1-, 2- or 3-family home (condos and LLC-owned homes ineligible; Downtown base-code districts excluded) under the Article 2 citywide text amendment (BPDA Board April 11, 2019). Detached and attached ADUs by right only in the Greater Mattapan R1 and R2 subdistricts inside the PLAN: Mattapan area (Article 60, Zoning Commission January 2024); elsewhere they need a ZBA variance. Check your subdistrict in the city's Zoning Viewer. |
| Max Size (Detached) | By right only in Mattapan R1/R2: building floor plate up to 900 sq ft or the floor plate of the main house, whichever is less (Article 60). Elsewhere the code has no detached-ADU rule, so any size needs a ZBA variance. |
| Max Size (Attached) | By right only in Mattapan R1/R2: an addition or bump-out may add up to 75% of the principal unit's gross square footage or 1,250 sq ft, whichever is smaller, and be no taller than the main house (Article 60). Elsewhere an attached ADU needs a ZBA variance. |
| Max Size (JADU) | Boston has no JADU category. The citywide by-right unit is the interior ADU: one self-contained unit with its own entrance built entirely within the existing footprint of an owner-occupied 1-3 family home, adding no square footage; the code sets no separate square-foot cap for it. Mattapan also defines an 'Extended Living Suite' (shared circulation, may share a bathroom) that is not a separate unit. |
| Max Height | Interior ADU: no change to the building. Mattapan R1/R2: a detached ADU no more than 1.5 stories or the height of the main house, whichever is less; an attached ADU no taller than the main house; a lot adding an ADU may build to 40 ft / 3.5 stories (Article 60). Elsewhere the subdistrict's height limit applies through the ZBA. |
| Max Units | One interior ADU per lot citywide. Mattapan R1/R2: up to one non-detached and one detached ADU, within a lot total of 3 dwelling units in R1 or 4 in R2 counting ADUs (Article 60, Table 7). |
Setback Requirements [1][2][3][4][5][6]
Boston requires a rear setback of about 20 ft for a detached ADU, and parking requirements vary by unit size or location. Owner-occupancy is required.
| Rear Setback | 20 ft in Mattapan R1/R2 for a lot adding an ADU (Article 60 dimensional table for ADU lots). Elsewhere the base subdistrict's rear yard applies and a detached ADU needs a ZBA variance. |
| Side Setback | 3 ft in Mattapan R1/R2 for a lot adding an ADU, and a detached ADU must sit at least 5 ft from any other building on the lot (Article 60). Elsewhere the base subdistrict's side yard applies. |
| Front Setback | 5 ft in Mattapan R1/R2 for a lot adding an ADU (the ordinary R1/R2 front yard is 10 ft) (Article 60). Elsewhere the base subdistrict's front yard applies. |
| Parking Required | No ADU-specific parking rule is published for an interior ADU; Mattapan's 2024 amendment set parking minimums to current Boston Transportation Department policy. Confirm the required count for your subdistrict with the ISD plans examiner - a parking shortfall is a common item in ZBA variance cases. |
| Owner Occupancy | Required: the owner must live on the parcel, which must be a 1-, 2- or 3-family home. Condominiums and LLC-owned properties are ineligible. The state ban on ADU owner-occupancy rules (G.L. c. 40A s. 3) does not reach Boston. |
| Min Lot Size | None for an interior ADU (existing building). Mattapan R1/R2 ADU lots: no minimum lot size; instead maximum building coverage 60% (lots under 5,000 sq ft) or 50% (5,000 sq ft and up), minimum 15% permeable area and 20 ft frontage (Article 60). |
Permit Process & Fees [7][8][9]
An ADU permit in Boston goes through a standard permit review. Typical timeline: ISD plan review takes about 5 weeks for a complete application (city ADU FAQ); a ZBA variance adds a hearing cycle. City guidance: design up to 3 months, permitting up to 3 months, construction 6-12 months. Permit fees: Long-form building permit $50 plus $10 per $1,000 of estimated construction cost (ISD fee schedule dated May 15, 2023): about $950 on a $90,000 interior ADU, $3,050 on a $300,000 detached ADU; ZBA appeal $150 for a 1-3 family home.
| Approval Process | Every ADU needs an ISD long-form building permit (online or at 1010 Massachusetts Ave) with architect-stamped plans, the contractor agreement and workers' comp certificate, plus electrical, plumbing, gas or sprinkler permits as needed. Interior ADUs and Mattapan R1/R2 exterior ADUs need no zoning relief. Other detached or attached ADUs get an ISD zoning refusal letter, then a ZBA appeal filed within 45 days ($150) and a public hearing. Register the finished unit with the city's annual rental registry. |
| Review Timeline | ISD plan review takes about 5 weeks for a complete application (city ADU FAQ); a ZBA variance adds a hearing cycle. City guidance: design up to 3 months, permitting up to 3 months, construction 6-12 months. |
| Permit Fees | Long-form building permit $50 plus $10 per $1,000 of estimated construction cost (ISD fee schedule dated May 15, 2023): about $950 on a $90,000 interior ADU, $3,050 on a $300,000 detached ADU; ZBA appeal $150 for a 1-3 family home. |
| Impact Fees | None: ISD's fee schedule lists no impact or capacity fee. Trade permits are small (electrical $20 plus $1 per fixture, plumbing $20 plus $5 per fixture, gas $20 plus $5 per appliance). Ask BWSC about connection charges if a detached ADU needs its own water or sewer service. |
| Plan Check | About 5 weeks for a complete long-form application (city ADU FAQ); longer if plans are revised, documents are missing or zoning relief is needed. |
| Pre-Approved Plans | Not available. The city's ADU Guidebook (November 2024) illustrates designs for common Boston house types but contains no pre-approved plans; every project needs its own architect-stamped drawings. |
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Construction Costs [10][11]
Building an ADU in Boston typically costs $75,000-$350,000 ($150-$400 per sq ft); a garage conversion runs $150,000-$275,000.
See what this means for your lot — ask Boston ADU builders for a quote →
| Average Build Cost | $75,000-$350,000 |
| Cost per Sq Ft | $150-$400 |
| Garage Conversion | $150,000-$275,000 |
| Prefab ADU (Installed) | $180,000-$320,000 |
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Rental Income Potential [12][13][14]
A 2-bedroom ADU in Boston rents for around $3,000-$3,500/mo, an estimated 7-12% gross annual yield (estimate).
| Avg Rent (1 Bedroom) | $2,500-$2,900/mo |
| Avg Rent (2 Bedroom) | $3,000-$3,500/mo |
| Short-Term Rental | Only through Boston's short-term rental registry, which requires owner-occupancy: a second unit in an owner-occupied 2-3 family home can be registered as 'owner-adjacent' ($200/yr). |
| Est. Annual ROI | 7-12% gross annual yield (estimate) |
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Recent Law Changes
What's Changed Recently
- September 20, 2026: entry re-verified against the city's ADU toolkit (boston.gov, read September 19, 2026), the Mattapan Article 60 fact sheet, ISD's fee schedule and HUD FY2027 rents. The unsourced interior-ADU size cap (900 sq ft / 50%), the $4,500-$7,500 permit figure and the $2,000-$3,500 impact fee were removed; real fees are $50 plus $10 per $1,000 of cost.
- July 17, 2026: the Planning Department released the DRAFT Neighborhood Housing residential zoning text amendment (first phase: Hyde Park, Roslindale and West Roxbury) that would allow ADUs without a variance; public meetings ran in late July 2026. It is not adopted and still needs BPDA Board and Zoning Commission votes.
- May 2026: the Planning Department's 'Enabling ADUs' citywide zoning page now points readers to the Neighborhood Housing initiative; the Zoning Commission's April and May 2026 hearings were cancelled, so nothing on ADUs was adopted this spring.
- The statewide ADU-by-right law (Affordable Homes Act, G.L. c. 40A s. 3, effective February 2, 2025) does not apply in Boston: the city zones under its own 1956 enabling act, and the city's ADU FAQ says so explicitly.
- January 2024: the Article 60 amendment made detached and attached ADUs by right in Mattapan's R1 and R2 subdistricts; the city reports 90% of small residential projects there are now by right, up from 60% before rezoning.
- ADU Financial Assistance Program (current terms): 0% deferred loan up to $50,000 for households up to 135% of AMI (1:1 match required above 120%) plus a $7,500 technical-assistance grant; applicants must attend the ADU Design and Budget workshops within 3 months before applying.
Tips for Building an ADU in Boston
- Cost basis: the city's own ADU FAQ puts an internal ADU at $75,000-$100,000 and a detached ADU at $250,000-$350,000, and the Boston Home Center uses $275 per sq ft as its rule of thumb (800 sq ft = $220,000); the garage-conversion and prefab ranges here are estimates from those figures, not city numbers. Rents: HUD FY2027 FMR for the Boston HMFA is $2,518 (1BR) and $3,008 (2BR); Zumper's Boston medians on September 20, 2026 were $2,875 and $3,495.
- Decide the type first: a basement or attic unit inside the existing footprint is a permit-only project citywide, while a backyard cottage or an addition outside Mattapan R1/R2 means a ZBA variance (refusal letter, appeal within 45 days, $150, public hearing) and no guarantee of approval.
- Mattapan owners: the by-right rules apply only if the Zoning Viewer shows your parcel in the Greater Mattapan Neighborhood district with an R1 or R2 subdistrict; the 2F-3000, 3F-5000 and 3F-6000 subdistricts outside the PLAN: Mattapan area were not changed.
- You must own and occupy the home; condominiums and LLC-owned properties are ineligible for any ADU, and the interior ADU cannot add gross square footage or change the exterior without going to the ZBA.
- Plans must be stamped by a licensed architect, and a licensed general contractor is required unless the property is a single-family home and you file the Homeowner Exemption form; in a FEMA flood zone the ADU must sit above the Base Flood Elevation.
- The building permit is valid for six months and work must not stop; a 180-day extension is sometimes possible. After the final inspection, register the ADU with the city every year even if a family member lives in it rent-free.
- Income-eligible owner-occupants can stack the $7,500 technical-assistance grant, the $50,000 0% deferred city loan and a partner-bank ADU loan; apply after you have an architect and two contractor quotes (quotes under $100,000 get extra scrutiny).
- Cost basis (2026-09-20 re-verification): short term allowed: Only through Boston's short-term rental registry, which requires owner-occupancy: a second unit in an owner-occupied 2-3 family home can be registered as 'owner-adjacent' ($200/yr). The city's STR page does not say whether an ADU qualifies - confirm with ISD before planning nightly rentals.
ADU Builders in Boston
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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.
- Research your ADU Options - ADU Toolkit, Mayor's Office of Housing (zoning for internal, attached, detached and Mattapan ADUs) Planning Department — Accessed 2026-09-19
- Frequently Asked ADU Questions - City of Boston (where ADUs can be built; Affordable Homes Act does not apply to Boston) Planning Department — Accessed 2026-09-19
- Article 60 Residential Zoning Amendment Fact Sheet - Boston Planning Department (Mattapan ADU definitions, Tables 5-7; adopted January 2024) Planning Department — Accessed 2026-09-20
- Neighborhood Housing - Boston Planning Department (citywide residential rezoning to enable ADUs; draft phase) Planning Department — Accessed 2026-09-20
- Enabling Accessory Dwelling Units - Boston Planning Department (citywide ADU zoning page; ADU Guidebook is not pre-approved plans) Planning Department — Accessed 2026-09-20
- The citywide additional dwelling unit program moves forward - BPDA, April 12, 2019 (Article 2 interior-ADU text amendment approved by the BPDA Board) Planning Department — Accessed 2026-09-20
- Permitting Your ADU - ADU Toolkit, Mayor's Office of Housing (long-form permit, 5-week review, ZBA $150 and 45-day filing window) Planning Department — Accessed 2026-09-19
- Inspectional Services Department - Building Permit Fees (schedule dated May 15, 2023) Planning Department — Accessed 2026-09-20
- ADU Financial Assistance Program - Boston Home Center (grant $7,500; 0% deferred loan up to $50,000; eligibility) Planning Department — Accessed 2026-09-20
- Frequently Asked ADU Questions - City of Boston (internal ADU $75,000-$100,000; detached $250,000-$350,000) Planning Department — Accessed 2026-09-19
- ADU Financial Assistance Program - Boston Home Center ($275 per sq ft rule of thumb) Planning Department — Accessed 2026-09-20
- HUD FY2027 Fair Market Rents - Boston-Cambridge-Quincy, MA-NH HMFA (1BR $2,518; 2BR $3,008) Government Data — Accessed 2026-09-20
- Zumper - Boston, MA rent research (median 1BR $2,875, 2BR $3,495 as of September 20, 2026) Market Data — Accessed 2026-09-20
- Short-Term Rentals - City of Boston (registration types and fees; owner-occupancy) Government Data — Accessed 2026-09-20
- Accessory Dwelling Units (ADUs) in Boston - Mayor's Office of Housing ADU Toolkit Planning Department — Accessed 2026-09-19
- Boston Zoning Commission - hearing calendar (April and May 2026 hearings cancelled) Planning Department — Accessed 2026-09-20
Frequently Asked Questions
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