Everything homeowners ask about building an ADU in Cambridge — answered with 2026 data.
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, MA. 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. Attached ADUs can be up to 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. Cambridge has an ADU-friendliness score of 9/10. Check our full Cambridge ADU rules page for complete zoning details.
ADU construction in Cambridge typically costs $220,000-$400,000. The cost per square foot averages $275-$450. Garage conversions are more affordable at $100,000-$250,000, while prefab ADUs cost $250,000-$480,000. Permit fees add 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. to your budget. See our detailed Cambridge ADU cost guide.
Detached ADUs in Cambridge can be up to No local size cap since Feb 10, 2025. Attached ADUs are limited to 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. Junior ADUs (JADUs) have a cap of 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. Maximum height is 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. You are allowed up to No local cap: the 'no more than one accessory apartment per lot' clause of old 4.22.1 on a qualifying lot.
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. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
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. Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
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. 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 Cambridge is typically 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. 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 availability: Not available. Cambridge publishes no ADU program page, pre-approved plan catalogue or ADU handbook (CDD and ISD sites checked 2026-09-20).
Average ADU rental income in Cambridge: 1-bedroom units rent for $3,200-$3,600/month, 2-bedroom units for $3,800-$4,300/month. Short-term rentals: 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. Expected ROI: 9-14% gross rent yield.
The minimum lot size for an ADU in Cambridge is 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). 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 Cambridge, a garage conversion typically costs $100,000-$250,000. 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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