Compare ADU zoning rules across 8 Massachusetts cities. Average ADU score: 7.0/10.
Gathered from the 8 cities below, with the bill or ordinance each one names. Statewide measures are listed once with the cities they reached, rather than repeated for each.
Statewide law sets a floor; the numbers that decide your project are local. Here is the spread across the 8 Massachusetts cities covered here, taken from each city's own ordinance rather than a state summary.
| Question | Across Massachusetts |
|---|---|
| Are ADUs permitted? | 7 of 8 permit them, 1 only by discretionary approval. |
| Detached size cap | Median 900 sq ft, from 320 to 900 sq ft across the 8 cities publishing a numeric cap. |
| Owner-occupancy | Required in 1 of 8. |
| Build cost | $75,000 to $450,000 depending on city and unit type. |
| Easiest and hardest here | Cambridge scores 9/10; Boston scores 5/10. |
Recomputed whenever a city page changes, so this and the city pages below cannot disagree.
| City | Max Size | Setbacks | Permit Fees | Build Cost | 1BR Rent | Score |
|---|---|---|---|---|---|---|
| Boston | 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. | 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. | 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. | $75,000-$350,000 | $2,500-$2,900/mo | 5/10 |
| Cambridge | 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. | 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)). | 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. | $220,000-$400,000 | $3,200-$3,600/mo | 9/10 |
| Medford | The smaller of 900 sq ft or half the principal dwelling's gross floor area. Medford § 94-8.2.3(4) and M.G.L. c. 40A § 1A / 760 CMR 71.02 agree on the number. Under 760 CMR 71.02, GFA counts every story of compliant ceiling height including basements and lofts but excludes crawl spaces, garages, attics and enclosed porches — so a 1,400 sq ft house gets a 700 sq ft unit. § 94-8.2.3(4) lets the Board exceed 900 sq ft only for a carriage house or barn with preservation value, by special permit. | 3 ft for a detached ADU, or 1 ft with fire-resistance-rated wall assemblies (§ 94-4.3.2(3), accessory structures). A dwelling needs a 15 ft rear yard (Table B, row 1), but 760 CMR 71.03(3)(b)(2) gives the ADU the more permissive of the two. An accessory building may occupy up to 25 percent of the rear yard (§ 94-4.2.5(1)) and must sit at least 10 ft from the house and from any other accessory building (§ 94-4.1.2(3)(a)). | $15.00 per $1,000 of estimated construction cost plus a $35.00 application fee, a $100.00 plan review fee for 1-4 family dwellings and $125.00 for a certificate of occupancy (Schedule of Building Fees effective July 28, 2009; Code Appendix A, § 10-35) — about $3,885 on a $250,000 ADU. | $220,000-$400,000 | $2,500-$2,800/mo | 7/10 |
| Newton | 1,000 sq ft or 50% of the combined gross floor area of house plus ADU, whichever is less, by right (Sec. 6.7.1.E.2); 1,001-1,200 sq ft by right if the unit is in an accessory building with a final permit sign-off at least 4 years old (E.2.c) or the owner lives in the house or ADU and files an annual certification (E.2.d); up to 1,500 sq ft by City Council special permit (E.2.b). These caps are local; the state Protected Use size is 900 sq ft or half the main house. | 5 ft for a detached ADU of no more than 1.5 stories and 22 ft (accessory-building setback, Sec. 3.4.3 via Sec. 6.7.1.E.4; Planning FAQ); a taller unit meets the single-family rear setback for the district (ISD Zoning Chart: 40 ft SR1, 30 ft SR2/SR3, 15-30 ft MR on post-1953 lots; 25 ft on older SR lots). The 1.5-story setback can be reduced by special permit (E.4). | $20 per $1,000 of estimated construction cost, rounded up, $50 minimum, for each of the building, electrical, plumbing and gas permits (ISD Permit Fee Schedule, updated Jan 1, 2026): about $6,000 for a $300,000 ADU building permit. Special-permit fees are separate. | $250,000-$450,000 | $2,600-$3,600/mo | 7/10 |
| Quincy | 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is less (760 CMR 71.02, applied by the ISD guideline). Minimum 320 sq ft for a detached ADU — a local ISD figure, not a state or 780 CMR rule. Example: a 1,500 sq ft house allows 750 sq ft; 1,800 sq ft or larger allows the full 900. | Detached ADU must meet the principal-building yards in QZO Table 4.1 (ISD guideline): Residence A 20 ft rear; Residence B (1-3 units) 25 ft rear; Residence C half the building height, never less than 25 ft. Note: a garage or shed may sit 6 ft from the rear line (§375-3.2.3.1) and 760 CMR 71.03(3)(b)2 bars ADU dimensional rules stricter than those for an accessory structure — ask ISD in writing which figure it will apply. | Valuation-based building permit fee under Municipal Code Ch. 150 Art. III (Inspectional Services fees); no ADU-specific fee is published. Confirm the current figure with ISD (617-376-1450) before budgeting. | $220,000-$400,000 | $2,250-$2,650/mo | 6/10 |
| Somerville | Local Backyard Cottage: 576 sq ft floor plate, max 24 ft wide x 32 ft deep, 1.5 stories (SZO §3.1.12.c). State Protected Use ADU: up to 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is smaller (760 CMR 71.02); a detached state ADU may be built to that cap, and city dimensional rules can be no stricter than those for the house or an accessory structure (71.03(3)(b)2). The pending Sept 2026 amendment would swap the 576 sq ft floor plate for a 1,750 sq ft GFA cap. | 3 ft minimum; 3 or 20 ft where the lot abuts an alley (SZO §3.1.12.b). 10 ft minimum separation between the cottage and the principal building. | Not published online. ISD charges a plan-review fee at application, credited toward the building permit fee if no plan review is needed; the building permit fee itself is set on the Estimated Cost Affidavit — confirm the current rate with ISD before budgeting. | $220,000-$400,000 | $2,900-$3,150/mo | 7/10 |
| Waltham | 450 to 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' (§ 3.23(b), Ord. No. 36552). Under 1,800 sq ft of house the half rule binds first; gross floor area follows 760 CMR 71.02. The 450 sq ft floor is Waltham's own; 760 CMR 71.02 allows 'additional size restrictions' subject to the 71.03(3)(a) test. | 5 ft from the rear lot line is the operative figure for a protected-use ADU: the accessory-building standard is 5 ft from rear and side lines and 10 ft from the house (§ 4.221), and 760 CMR 71.03(3)(b)(2) bars any ADU setback stricter than the principal-dwelling or accessory-structure standard, 'whichever results in more permissive regulation'. On its face § 3.23(c) applies the district rear yard — 40 ft in RA-1/RA-2, 30 ft in RA-3/RA-4/RB, 20 ft in RC/HR1, 60 ft in RD (Table 4.11). The 10-ft separation and 25%-of-rear-yard cap still apply. | Residential building permits are $12 per $1,000 of estimated construction cost, $50 minimum (about $3,000 on a $250,000 ADU); plumbing is $25 for the first fixture and $10 each after, gas $30 plus $15 per added appliance, and a residential trench permit $50 (Building Department, Sept. 2026). | $200,000-$400,000 | $2,550-$3,050/mo | 8/10 |
| Worcester | 900 sq ft gross floor area (Section 8(G)(5)(a)); larger by Planning Board special permit (Section 8(G)(7)). GFA counts every story of code-compliant ceiling height, including basements and lofts, measured from the interior face of exterior walls, and excludes crawl spaces, garage parking, attics and enclosed porches (Section 8(G)(2)). Worcester applies no 50%-of-principal-dwelling test, so the local 900 sq ft cap is more permissive than the state Protected Use ADU definition. | 5 ft, via the accessory-structure standard in Section 8B(10) (no accessory building closer than 5 ft to any lot line), which Section 8(G)(5)(b)i lets an ADU use because it is more permissive than the Table 4.2 principal-building rear yard (20 ft in RS-10, RS-7 and RL-7; 15 ft in RG-5). Setbacks that cannot be met may be modified by Planning Board special permit (Section 8(G)(7)). | $12 per $1,000 of construction value (minimum $100) plus a $100 certificate of occupancy, per the ISD Schedule of Fees (2024) - about $2,500-$4,400 on a $200,000-$360,000 ADU; a Planning Board special permit is $330. | $220,000-$360,000 | $1,450-$1,750/mo | 7/10 |
Tell us about your project and we'll pass it to ADU builders serving Massachusetts. Free, no obligation — you choose who to work with. We don't have builders in every area yet; if nobody covers yours, we'll email you to say so.
🔒 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.
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