Can I build an ADU in Cambridge, Massachusetts?
Yes. Cambridge's Zoning Ordinance permits Accessory Apartments in every zoning district where residences are allowed, with no special permit, and it sets no owner-occupancy rule or size cap. The local definition covers a unit inside a single- or two-family house; detached ADUs aren't addressed locally, but state law has protected one attached or detached ADU per lot since February 2, 2025.
| Where allowed | All districts where residences are permitted [1] |
| Approval type | Permitted (no special permit in the ordinance text) [1] |
| Local definition | Unit with its own kitchen and bath, located within a single- or two-family detached dwelling [2] |
| Ownership | Under the same ownership as the main dwelling [2] |
| Converting an accessory building | Must meet the district dimensional rules (Section 5.30) [3] |
| Owner-occupancy requirement | None in the ordinance text [1] |
Where ADUs are allowed
Section 4.22 is a single sentence: Accessory Apartments are permitted in all zoning districts where Residences are permitted [1]. The ordinance doesn't add a special permit, owner-occupancy rule, minimum lot size or size cap for them.
What counts as an Accessory Apartment
Article 2 defines an Accessory Apartment as a dwelling unit with its own kitchen and bathroom, located within and under the same ownership as a single- or two-family detached dwelling and used by one household [2]. In other words, the local text describes an interior unit (basement, attic or part of the house), and it must stay in the same ownership as the main home.
Detached units and garage conversions
Cambridge's ordinance doesn't separately regulate detached ADUs. An existing accessory building (such as a garage) can't be converted to residential use unless it meets the district dimensional regulations in Section 5.30 [3]; in the Residence C-1 district, Table 5-1 sets a 10-foot front and 5-foot side/rear setback for residential uses, and accessory buildings there are limited to 15 feet in height [4]. How the city classifies a new detached ADU (accessory building or second dwelling) isn't spelled out, so confirm which height and setback rules apply. Since February 2, 2025, Massachusetts law (M.G.L. c. 40A, §3) has protected one attached or detached ADU per lot of up to 900 sq ft or half the main home's floor area (whichever is smaller), so state law may override the narrower "located within" definition for a detached ADU. The city can still apply reasonable dimensional rules such as setbacks and height.
Short-term rental of an ADU
An ADU is a dwelling unit, so Cambridge's short-term rental rules in Section 4.60 apply: only operator-occupied and owner-adjacent rentals are permitted, and every operator must register with Inspectional Services [5].
Parking and permits
The off-street parking rules (Article 6) weren't among the sections we reviewed, so check parking with the city. Building work needs a building permit from the Inspectional Services Department, and the work must comply with the Zoning Ordinance [6].
See how Cambridge compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →
Next steps
- Confirm your lot's zoning district on the city zoning map (Community Development Department).
- For a detached or garage ADU, ask Inspectional Services how it applies the state ADU law alongside Section 4.21 and Section 5.30.
- Apply for a building permit from the Inspectional Services Department, 831 Massachusetts Ave (617-349-6100), with plans showing the separate kitchen, bathroom and entrance.
- If the house is in a historic or neighborhood conservation district, get Cambridge Historical Commission approval first.
Common questions
- Can I build an ADU in Cambridge, MA?
- Yes. Cambridge's Zoning Ordinance permits Accessory Apartments in every zoning district where residences are allowed, with no special permit. You still need a building permit from Inspectional Services, and historic or conservation districts may need Historical Commission approval first.
- Is there an owner-occupancy or size limit for ADUs in Cambridge, Massachusetts?
- No. The Cambridge ordinance sets no owner-occupancy rule or size cap, though the apartment must stay under the same ownership as the main dwelling.
- Can I build a detached ADU in Cambridge, MA?
- It's not addressed locally. Cambridge's definition covers a unit inside a single- or two-family house, but state law since February 2, 2025 protects one attached or detached ADU per lot, so it may override that. Converting a garage must meet Section 5.30 dimensional rules; ask Inspectional Services.
- Can I Airbnb my ADU in Cambridge, Massachusetts?
- Only under the city's short-term rental rules. An ADU is a dwelling unit, so only operator-occupied and owner-adjacent rentals are permitted, and every operator must register with Inspectional Services.
Massachusetts' statewide ADU law
Since February 2, 2025, the Affordable Homes Act (St. 2024, c. 150, §8) has protected one accessory dwelling unit (ADU) per lot in any zoning district that allows single-family homes (M.G.L. c. 40A, §3). Your town cannot ban it, require a special permit or other discretionary approval, or "unreasonably restrict" building or renting it.
What counts as a protected ADU:
- A self-contained unit with its own sleeping, cooking and bathroom facilities, on the same lot as the main home.
- No larger than 900 sq ft or half the main home's floor area, whichever is smaller.
- It can be attached or detached, and it needs a separate entrance that meets the building code.
What towns cannot require:
- That the owner live in the main home or the ADU.
- More than 1 extra parking space, or any extra space if the lot is within 0.5 miles of a subway, commuter rail, ferry or bus station.
- A minimum lot size, caps on the number of ADUs, or limits on who may live there (age, income, relationship).
What towns can still do: use reasonable site plan review, setback, height and bulk rules, require Title 5 septic compliance, and restrict or ban short-term rentals of ADUs. A second ADU on the same lot needs a special permit. State regulations at 760 CMR 71.00 (in effect since January 31, 2025) explain which rules count as unreasonable.
What the bylaw says
- Sec. 4.22, Zoning Ordinance Article 4.000 Use Regulations (4.21 Accessory Uses incl. home occupations and accessory buildings; 4.22 Accessory Apartments; 4.30 use table; 4.60 Short-Term Rentals), p. 1
Accessory Apartments shall be permitted in all zoning districts where Residences are permitted.
- Art. 2.000, Accessory Apartment, Zoning Ordinance Article 2.000 Definitions (Accessory Apartment, Short-Term Rental, Structure, etc.), p. 1
An accessory use with one or more rooms with separate kitchen and bathroom facilities, constituting a dwelling unit, located within and under the same ownership as a single or two-family detached dwelling and used as the residence of one residential household.
- Sec. 4.21(l), Zoning Ordinance Article 4.000 Use Regulations (4.21 Accessory Uses incl. home occupations and accessory buildings; 4.22 Accessory Apartments; 4.30 use table; 4.60 Short-Term Rentals), p. 1
No accessory building may be converted to a residential use unless it conforms with the district dimensional regulations specified in Section 5.30
- Sec. 4.21(k), Zoning Ordinance Article 4.000 Use Regulations (4.21 Accessory Uses incl. home occupations and accessory buildings; 4.22 Accessory Apartments; 4.30 use table; 4.60 Short-Term Rentals), p. 1
An accessory building in Residence C-1, and Office-1 districts shall not exceed fifteen (15) feet in height above the ground level.
- Sec. 4.64, Zoning Ordinance Article 4.000 Use Regulations (4.21 Accessory Uses incl. home occupations and accessory buildings; 4.22 Accessory Apartments; 4.30 use table; 4.60 Short-Term Rentals), p. 1
Only operator-occupied short-term rentals and owner-adjacent short-term rentals are permitted.
- ISD FAQ: What requirements must be met prior to applying for a building permit?, ISD Building/Electrical/Plumbing Permit FAQ
The work must be in compliance with the Cambridge Zoning Ordinance.