ADU rules in Cambridge

The city's own Accessory Apartment use covers only units inside a single- or two-family dwelling, so a detached backyard ADU proceeds under the statewide protected-use ADU right instead: one attached or detached ADU per lot, by right, in any district where single-family dwellings are a permitted or allowable use (which in Cambridge is every district that allows residences). State size cap applies: 900 sq ft or half the principal dwelling's gross floor area, whichever is smaller.

Section 4.22 (current text, one sentence): 'Accessory Apartments shall be permitted in all zoning districts where Residences are permitted.' An Accessory Apartment is defined as a dwelling unit with separate kitchen and bathroom located within, and under the same ownership as, a single- or two-family detached dwelling.

The pre-reform s.4.22 caps (900 sq ft or 35% of the principal dwelling, with an 1,800 sq ft principal-dwelling prerequisite) no longer appear anywhere in the current code. General accessory-use area limits in s.4.21.g (accessory uses up to 25% of building gross floor area; accessory buildings up to 15% of lot area) could still be read to constrain some projects.

Neither s.4.22 nor the Accessory Apartment definition requires owner occupancy; the definition only requires the unit to be under the same ownership as the principal dwelling (no separate sale). State law independently prohibits owner-occupancy requirements for protected-use ADUs (760 CMR 71.03(2)(a); MGL c.40A s.3).

Section 6.31: 'Notwithstanding any other provision in this Zoning Ordinance, the minimum accessory parking required for all uses shall be zero (0) parking spaces, including in all overlay districts.' The state protected-ADU parking limits (max 1 space; none within 0.5 mile of a transit station) are therefore moot in Cambridge.

By right: s.4.22 says accessory apartments 'shall be permitted' in all districts where residences are permitted, with no special permit or discretionary approval attached; state law also forbids requiring a special permit for a protected-use ADU. Standard building permit through Inspectional Services still applies.

Section 4.64 permits only operator-occupied and owner-adjacent short-term rentals, with registration through the Inspectional Services Department. An accessory apartment could qualify as an owner-adjacent STR only if the operator's primary residence is in the same building, the building has four or fewer units, and all units are owned by the operator (s.4.62.e). One STR dwelling unit of each type per operator (s.4.64.5).