ADU rules in Somerville

Somerville allows a detached backyard unit as a building type rather than as a use. Section 3.1.6 of the Somerville Zoning Ordinance permits one principal building and one accessory building per lot in the Neighborhood Residence (NR) district and lists the Backyard Cottage as an accessory building type permitted by right; section 3.1.12 gives it a maximum of one dwelling unit. The Affordable Housing overlay repeats the permission for NR land (section 8.1.4.a.i). The Urban Residence (UR) district's building-type section lists no accessory building types at all, but UR allows the one-unit Cottage principal type, which makes it a Single-family Residential Zoning District under 760 CMR 71.02, so the statewide protected-use ADU right reaches UR lots even though the local text is silent; SZO section 9.1.4 independently defers to anything protected by M.G.L. c. 40A section 3.

The zoning ordinance has no attached or internal ADU provision: the phrase 'accessory dwelling unit' appears nowhere in the SZO, and its Backyard Cottage is detached by definition. Attached and internal units instead run on two other tracks. First, the statewide protected-use right covers them: 760 CMR 71.03(2)(e) makes it a prohibited regulation to require that a protected ADU 'be attached to or detached from the Principal Dwelling', and SZO section 9.1.4 says the ordinance 'does not restrict the use of land or structures for activities protected by M.G.L. Chapter 40A, Section 3.' Second, many owners never need the ADU pathway: a Detached House in NR may hold one to three vertically stacked dwelling units by right (section 3.1.8), so a basement or floor conversion is often just an ordinary additional dwelling unit.

Section 3.1.12's massing table caps the Backyard Cottage at a 576 sq ft floor plate, 24 ft width, 32 ft depth, 12 ft story height, and 1.5 stories, with one dwelling unit maximum. Floor plate is defined at section 2.1.1 as 'the total gross floor area of a single story of a building, excluding building components', and section 2.4.4.a.viii counts habitable space under a pitched roof as the half story, so total finished area lands somewhere above 576 sq ft but well under a full two floors. The state floor for a protected ADU is 900 sq ft or half the principal dwelling, whichever is smaller (M.G.L. c. 40A section 1A), and c. 40A section 1A(iii) lets municipalities add size restrictions while 760 CMR 71.03(3)(b)(2) forbids dimensional standards more restrictive than those for the principal dwelling or an accessory structure, whichever is more permissive; Somerville has not codified how the 576 sq ft cap interacts with that.

Nothing in the SZO conditions a Backyard Cottage or any dwelling unit on owner occupancy. Section 10.2.1.d.iii requires only that an accessory structure be 'operated and maintained under the same ownership as the principal building', which is a common-ownership rule (no separate sale), not a residency rule. State law independently forbids owner-occupancy conditions on a protected ADU: c. 40A section 3 says the use 'shall not require owner occupancy of either the accessory dwelling unit or the principal dwelling', and 760 CMR 71.03(2)(a) lists owner-occupancy requirements as prohibited. The one place occupancy does bite is short-term rental licensing, where the rented unit must be the operator's own primary residence (Code of Ordinances section 7-253(b)).

Ord. 2024-15, adopted December 12, 2024 and titled 'Parking Minimums (Effective February 1, 2025)', stripped the minimums out of every district table. In the Neighborhood Residence district, Table 3.1.18 now shows '--' for all residential use categories in the only motor-vehicle column, and section 3.1.18.a.iii states flatly that 'There are no parking requirements for accessory uses.' Table 3.2.17 reads the same way for Urban Residence. The state ceilings for a protected ADU (at most one space, and none within half a mile of a subway, commuter rail, ferry, or bus station) therefore never come into play here.

Section 3.1.6.c permits the Backyard Cottage by right in the Neighborhood Residence district, and section 10.2.1.a repeats that 'Unless otherwise specified, accessory structures are permitted by-right.' No Special Permit or Site Plan Approval is attached to the building type, so review is the ordinary Building Permit from the Building Official, who must issue it or give written reasons within 30 days of a complete application (section 15.3.5.d). State law bars a special permit or other discretionary approval for the first protected ADU on a lot in any single-family residential zoning district.

Zoning says only that 'Rental of a dwelling unit for less than one (1) month requires compliance with all City Ordinances' (section 9.2.11.a.i), which routes short stays to Code of Ordinances ch. 7, art. X (Ord. No. 2019-12 as amended by Ord. No. 2023-01). That article defines a short-term rental as fewer than 28 consecutive days, requires a certificate of registration from Inspectional Services ($250, expiring each March 31, registration in force since April 1, 2023), and requires that the rented unit 'shall be the operator's primary residence.' A whole-unit ('autonomous unit') rental is capped at 90 days a year; a 'shared unit' where the operator stays on site may run 365 days. The city's own page puts the ADU-relevant consequence plainly: 'you cannot offer an adjacent unit as a short-term rental.' Long-term rental is unrestricted, and c. 40A section 3 protects the rental of a protected ADU, while 760 CMR 71.03(3)(c) preserves municipal power to restrict short-term rental. Separate sale is barred by the same-ownership rule at section 10.2.1.d.iii.