Building an ADU in Cambridge, Massachusetts
City code and Massachusetts law · Last reviewed October 9, 2026
Cambridge's zoning allows an accessory apartment in every district where homes are allowed, and it requires no off-street parking for any use. Since February 10, 2025, almost all residential neighborhoods are zoned Residence C-1, which sets no minimum lot size and no maximum number of units, and allows homes up to four stories. The state's protected-use ADU law also applies here, because Cambridge zones under Chapter 40A. You still need a building permit from Inspectional Services. [1][2][3][4][5]
| Accessory apartments allowed | In all zoning districts where residences are permitted [1] |
|---|---|
| Accessory apartment (city definition) | A dwelling unit with its own kitchen and bathroom inside a single- or two-family detached house, same ownership [6] |
| Base residential zoning | Residence C-1 in almost all neighborhoods since Feb. 10, 2025 [3][7] |
| Unit limits and lot size | No minimum lot size, maximum number of units or maximum floor area for housing [3] |
| Height (Res. C-1) | 4 stories / 45 ft for residential uses; up to 6 stories with 20% inclusionary housing on lots of 5,000+ sq ft [7][10] |
| Setbacks (Res. C-1, residential) | 10 ft front; 5 ft side and rear [10] |
| Open space (Res. C-1) | At least 30% of the lot [10] |
| Parking | Zero minimum for all uses [2] |
| Building permit fee | $15 per $1,000 of construction cost for buildings with three or fewer units ($50 minimum) [13] |
| State protected-use ADU law | Applies; Cambridge zoning is controlled by M.G.L. c. 40A [4][5] |
| Short-term rental | Only operator-occupied or owner-adjacent rentals, registered with Inspectional Services [1] |
On this page
What you can build in Cambridge
Section 4.22 of the Cambridge Zoning Ordinance is one sentence: "Accessory Apartments shall be permitted in all zoning districts where Residences are permitted."
The ordinance defines an accessory apartment as 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."
Cambridge also went much further in 2025. On February 10, 2025, the City Council adopted citywide multifamily zoning. Almost all residential neighborhoods (the former Residence A-1, A-2, B and C districts) are now Residence C-1, and "all types of housing can be built" there, from single-family homes to multifamily buildings. Zoning no longer sets a minimum lot size or a maximum number of units. So adding a unit is no longer limited to the accessory apartment rules; confirm the best path for your lot with Inspectional Services.
See our Massachusetts ADU guide and what is an ADU. [1][3][6][7]
How state ADU law applies
Cambridge's Inspectional Services Department says the city's zoning "is controlled by MGL 40A." That means the state's protected-use ADU rules, in effect since February 2, 2025, set a floor here, unlike in Boston.
Under M.G.L. c. 40A, Section 3, zoning may not "prohibit, unreasonably restrict or require a special permit" for a single ADU in a single-family residential zoning district, cannot require owner occupancy, and can require at most one extra parking space (none within half a mile of a subway, commuter rail, ferry or bus station). State law caps a protected ADU at half the main home's gross floor area or 900 sq ft, whichever is smaller.
Cambridge's own rules are generally more permissive: it requires no parking at all, and its 2025 zoning sets no unit cap. The state housing office says it cannot answer questions about a specific property and tells owners to contact their city. [2][3][4][5][8][9]
Height, setbacks and open space
In Residence C-1, the dimensional table for residential uses sets:
- a maximum of 4 stories and 45 feet;
- a 10-foot minimum front yard and 5-foot minimum side and rear yards;
- a minimum open space ratio of 30%, with at least half of that meeting the private open space standards and at least half permeable.
Residential buildings can reach 6 stories (74 feet) if at least 20% is permanently affordable inclusionary housing and the lot is at least 5,000 sq ft. The city says a meeting with neighbors is required for a building of four or more stories.
Accessory buildings in Residence C-1 may not be nearer to any lot line than the district's minimum setback and may not exceed 15 feet in height. [1][3][7][10]
Parking
Cambridge requires no parking for an ADU or anything else. Section 6.31 says "the minimum accessory parking required for all uses shall be zero (0) parking spaces, including in all overlay districts." That is more generous than the state ADU law, which lets towns require one space. [2][5]
Converting a garage or carriage house
Cambridge's ordinance says no accessory building "may be converted to a residential use unless it conforms with the district dimensional regulations specified in Section 5.30." In Residence C-1 that generally means meeting the setbacks, height and open space limits above.
If an existing garage or carriage house doesn't conform, you may need relief from the Board of Zoning Appeal. A variance is needed if the ordinance prohibits what you propose; a special permit is needed where the ordinance calls for one. Separately, state regulations protect a single ADU in an existing nonconforming structure in many cases; ask Inspectional Services how that applies to your building. Our garage conversion guide covers the general steps. [1][4][11]
Permits and fees
Inspectional Services reviews every building permit application for zoning compliance before it issues the permit. Apply online through the city's permitting system, choosing "New Construction" for a new building or "Addition / Alteration" for work on an existing one, from the Building Permit Applications page.
The building permit fee is $20 per $1,000 of construction cost, but $15 per $1,000 for buildings with three or fewer residential units, with a $50 minimum. Plan review is included. A residential Certificate of Occupancy costs $100 for the first unit and $50 for each additional unit. If work starts before you get a permit, the fee is tripled. See the building fees page.
Inspectional Services is at 831 Massachusetts Avenue; call 617-349-6100. See our ADU permit process guide and ADU cost guide. [4][12][13]
Renting it out and short-term rentals
Long-term rental of an ADU is allowed. For a protected-use ADU under state law, the city cannot require you to live on the property.
Short-term rentals (under 30 consecutive days) are more limited. Cambridge permits only two types:
- operator-occupied: the unit is the operator's primary residence (up to three bedrooms may be rented separately);
- owner-adjacent: a unit in a building of four or fewer units, all owned by the operator, where the operator lives in another unit.
An operator may offer one unit of each type. Every operator must register with Inspectional Services before renting and carry liability insurance. [1][5]
Permit steps in Cambridge
Check your zoning district
Most residential lots are now Residence C-1. Confirm your district on the city's adopted zoning map, linked from the multifamily zoning page. [3]
Design to Residence C-1 limits
Plan for 5-foot side and rear setbacks, a 10-foot front setback, 30% open space and the height limit. A converted accessory building must meet the district dimensional rules. [1][10]
Apply for a building permit online
Use the New Construction or Addition / Alteration application on the Building Permit Applications page. Inspectional Services checks zoning compliance first. [4][12]
Go to the Board of Zoning Appeal if needed
If your plan doesn't comply, apply for a variance or special permit through the online permitting system. The BZA posts tentative 2026 hearing dates, including October 15 and 29, November 19 and December 17. [4]
Pay fees, build and get the Certificate of Occupancy
Pay the building permit fee ($15 per $1,000 for buildings of three or fewer units). After inspections, the residential Certificate of Occupancy is $100 for the first unit. [13]
Who to call
-
Inspectional Services Department
Building permits, zoning review, Board of Zoning Appeal applications, short-term rental registration
831 Massachusetts Avenue, Cambridge, MA 02139
-
Community Development Department (zoning policy)
Citywide multifamily zoning, zoning maps and design guidelines
Questions people ask
Are ADUs allowed in Cambridge?
Yes. Accessory apartments are permitted in every zoning district where residences are allowed, and since 2025 Residence C-1 zoning sets no maximum number of units. [1][3]
Do I need a parking space for an ADU in Cambridge?
No. Cambridge's minimum parking requirement is zero spaces for all uses. [2]
Do I have to live on the property?
Not for long-term rental: state law bars owner-occupancy requirements for a protected ADU. Short-term rentals in Cambridge, though, must be operator-occupied or owner-adjacent. [1][5]
How close to the lot line can I build?
In Residence C-1, residential buildings need 5 feet from side and rear lot lines and 10 feet from the front. Accessory buildings may not be nearer any lot line than the district minimum. [1][10]
How much is a building permit in Cambridge?
For buildings with three or fewer residential units, $15 per $1,000 of construction cost, with a $50 minimum. Plan review is included. [13]
Sources (13)
- Zoning Ordinance, Article 4.000: Use Regulations (Sections 4.21, 4.22, 4.60), City of Cambridge (Municode). Accessed October 9, 2026.
- Zoning Ordinance, Article 6.000: Off Street Parking (Section 6.31), City of Cambridge (Municode). Accessed October 9, 2026.
- Zoning for Multifamily Housing, City of Cambridge Community Development Department. Accessed October 9, 2026.
- Zoning, City of Cambridge Inspectional Services Department. Accessed October 9, 2026.
- General Laws, Part I, Title VII, Chapter 40A, Section 3, Massachusetts Legislature. Accessed October 9, 2026.
- Zoning Ordinance, Article 2.000: Definitions, City of Cambridge (Municode). Accessed October 9, 2026.
- Cambridge Zones for Multifamily Housing Citywide, City of Cambridge Community Development Department. Accessed October 9, 2026.
- General Laws, Part I, Title VII, Chapter 40A, Section 1A, Massachusetts Legislature. Accessed October 9, 2026.
- Accessory Dwelling Units, Executive Office of Housing and Livable Communities. Accessed October 9, 2026.
- Zoning Ordinance, Article 5.000: Development Standards (Section 5.31 table and footnotes), City of Cambridge (Municode). Accessed October 9, 2026.
- 760 CMR 71.00: Protected Use Accessory Dwelling Units, Executive Office of Housing and Livable Communities. Accessed October 9, 2026.
- Building Permit Applications, City of Cambridge Inspectional Services Department. Accessed October 9, 2026.
- Building Fees, City of Cambridge Inspectional Services Department. Accessed October 9, 2026.