Everything homeowners ask about building an ADU in Boston — answered with 2026 data.
Sometimes — but not by right. 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. Zoning: 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. Because approval is discretionary, confirm your own parcel with the planning department before budgeting. See our full Boston ADU rules page.
ADU construction in Boston typically costs $75,000-$350,000. The cost per square foot averages $150-$400. Garage conversions are more affordable at $150,000-$275,000, while prefab ADUs cost $180,000-$320,000. Permit fees add 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. to your budget. See our detailed Boston ADU cost guide.
Detached ADUs in Boston can be up to 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. Attached ADUs are limited to 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. Junior ADUs (JADUs) have a cap of 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. Maximum height is 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. You are allowed up to 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 on a qualifying lot.
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. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
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. Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
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. Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Boston is typically 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. 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 availability: 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.
Average ADU rental income in Boston: 1-bedroom units rent for $2,500-$2,900/month, 2-bedroom units for $3,000-$3,500/month. Short-term rentals: 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). Expected ROI: 7-12% gross annual yield (estimate).
The minimum lot size for an ADU in Boston is 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). This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Boston, a garage conversion typically costs $150,000-$275,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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