Everything homeowners ask about building an ADU in Somerville — answered with 2026 data.
Yes, ADUs are allowed in Backyard Cottage by right in the Neighborhood Residence (NR) district as the only permitted accessory building type (SZO §3.1.6.c, §3.1.12; Ord. 2023-23); the Urban Residence (UR) district takes the NR building types — confirm cottage eligibility on a UR lot with ISD Zoning. Separately, a state Protected Use ADU is by right on any lot in a district where single-family homes are allowed (NR and UR), including lots with a two- or three-family house (760 CMR 71.02, 71.03(1)). in Somerville, MA. The maximum size for a detached ADU is Local Backyard Cottage: 576 sq ft floor plate, max 24 ft wide x 32 ft deep, 1.5 stories (SZO §3.1.12.c). State Protected Use ADU: up to 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is smaller (760 CMR 71.02); a detached state ADU may be built to that cap, and city dimensional rules can be no stricter than those for the house or an accessory structure (71.03(3)(b)2). The pending Sept 2026 amendment would swap the 576 sq ft floor plate for a 1,750 sq ft GFA cap. Attached ADUs can be up to State cap only: 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is smaller. Somerville has an ADU-friendliness score of 7/10. Check our full Somerville ADU rules page for complete zoning details.
ADU construction in Somerville 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 Not published online. ISD charges a plan-review fee at application, credited toward the building permit fee if no plan review is needed; the building permit fee itself is set on the Estimated Cost Affidavit — confirm the current rate with ISD before budgeting. to your budget. See our detailed Somerville ADU cost guide.
Detached ADUs in Somerville can be up to Local Backyard Cottage: 576 sq ft floor plate, max 24 ft wide x 32 ft deep, 1.5 stories. Attached ADUs are limited to State cap only: 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is smaller. Junior ADUs (JADUs) have a cap of No JADU category in Massachusetts law or the Somerville ordinance; a small unit carved from the house is simply an ADU under the state 900 sq ft / 50% cap. Maximum height is Backyard Cottage: 1.5 stories max, 12 ft max story height, flat or gable roof only (SZO §3.1.12.c); dormers, bays, side wings and rear additions are not permitted on a cottage (Table 3.1.13, Ord. 2026-05). For a state ADU the city may not impose height or story limits stricter than those for the house or an accessory structure (760 CMR 71.03(3)(b)2) — confirm with ISD Zoning. You are allowed up to One cottage per lot on a qualifying lot.
Rear setback: 3 ft minimum; 3 or 20 ft where the lot abuts an alley (SZO §3.1.12.b). 10 ft minimum separation between the cottage and the principal building. Side setback: 3 ft minimum (SZO §3.1.12.b). Front setback: Primary front setback 60 ft minimum — the cottage sits in the rear yard; secondary (corner) front setback 10 ft (SZO §3.1.12.b). Minimum lot width for a cottage is 32 ft, or 34 ft with front driveway access (§3.1.12.a). The pending Sept 2026 amendment would cut the 60 ft front setback to 10 ft and delete the 10 ft building-separation rule. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None. Somerville requires no motor-vehicle parking for residential uses in the NR district and none for accessory uses (SZO §3.1.18.a.iii and Table 3.1.18, Ord. 2024-15); state law also bars any ADU parking requirement within 0.5 mile of a transit station, defined to include any MBTA bus stop (760 CMR 71.02, 71.03(2)(b)). 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. An owner-occupancy condition on a Protected Use ADU or its principal dwelling is a prohibited regulation statewide (760 CMR 71.03(2)(a)), and the Somerville ordinance imposes none on the Backyard Cottage. (A Council order of Jan 22, 2026, item 26-0100, only asked to discuss an incentive pilot for owner-occupied properties.). 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 Somerville is typically Not published. ISD posts no review-time target; a plan-review fee is paid when you apply and full plan review precedes issuance — ask ISD (617-625-6600 x5600) for the current queue. Plan check: Full architectural drawings and a site plan showing the 3 ft ambulance-cot walkway, buried utility route and the sprinkler design where required (SZO §3.1.12.f); Fire Prevention Bureau sign-off on the sprinkler exemption for corner, driveway or alley lots. Pre-approved plans availability: None. Somerville offers no pre-approved or pre-reviewed ADU plan library; the only related Council action is a Jan 2026 order to discuss an incentive pilot (item 26-0100).
Average ADU rental income in Somerville: 1-bedroom units rent for $2,900-$3,150/month, 2-bedroom units for $3,450-$3,750/month. Short-term rentals: No for a rented ADU. Expected ROI: 9-14% gross rent yield (estimate).
The minimum lot size for an ADU in Somerville is No minimum lot area. Cottage lots need 32 ft minimum width (34 ft with front driveway access) and lot coverage and Green Score are set by the principal building type, e.g. 60% coverage for a Detached House (SZO §3.1.8, §3.1.12.a). The state bars any minimum lot size for a Protected Use ADU (760 CMR 71.03(3)(b)2). 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 Somerville, 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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