Everything homeowners ask about building an ADU in Somerville — answered with 2026 data.
Yes, ADUs are allowed in By right in Neighborhood Residence (NR) district as the 'Backyard Cottage' accessory building type (SZO §3.1.12); state law (760 CMR 71) additionally protects one ADU by right on lots with a single-family dwelling in any district where single-family homes are permitted in Somerville, MA. The maximum size for a detached ADU is Local backyard cottage: 576 sq ft max floor plate, max massing 24 ft wide x 32 ft deep; state protected-use ADU: up to 900 sq ft or 50% of principal dwelling GFA, whichever is smaller (reasonable local dimensional standards still apply). Attached ADUs can be up to 900 sq ft or 50% of principal dwelling gross floor area, whichever is smaller (state cap); internal conversions (basement/attic) follow the same cap. 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 $275,000-$450,000. The cost per square foot averages $400-$600. Garage conversions are more affordable at $150,000-$250,000, while prefab ADUs cost $180,000-$350,000 installed (site work, foundations, and buried utilities extra — confirm with manufacturer). Permit fees add Confirm with Inspectional Services — building permit fees are valuation-based; budget several thousand dollars on a typical $300K+ build to your budget. See our detailed Somerville ADU cost guide.
Detached ADUs in Somerville can be up to Local backyard cottage: 576 sq ft max floor plate, max massing 24 ft wide x 32 ft deep; state protected-use ADU: up to 900 sq ft or 50% of principal dwelling GFA, whichever is smaller (reasonable local dimensional standards still apply). Attached ADUs are limited to 900 sq ft or 50% of principal dwelling gross floor area, whichever is smaller (state cap); internal conversions (basement/attic) follow the same cap. Junior ADUs (JADUs) have a cap of No JADU category in Massachusetts law. Maximum height is Backyard cottage: 1.5 stories max, ground story height 12 ft max, flat or gable roof only (SZO Table 3.1.12). You are allowed up to 1 dwelling unit per backyard cottage; if the cottage creates a 3rd unit on the lot, that unit must be permanently income-restricted (Tier 3 affordable, SZO Table 3.1.11/§12.1) — a state protected-use ADU added to a single-family home (2nd unit) does not trigger this on a qualifying lot.
Rear setback: 3 ft minimum (3 or 20 ft where lot abuts an alley); 10 ft minimum separation from the principal building. Side setback: 3 ft minimum. Front setback: Backyard cottages sit behind the principal building; primary front setback 32-34 ft depending on driveway configuration (SZO Table 3.1.12). Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None required for the ADU — state law bars parking requirements within 1/2 mile of a subway/rail/bus station, which covers virtually all of Somerville (Davis Red Line + 5 Green Line Extension stations). 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 allowed — prohibited statewide by the Affordable Homes Act effective Feb 2, 2025. 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 Building-permit plan review typically 4-8 weeks — Confirm with Inspectional Services (617-625-6600 ext. 5600, isd@somervillema.gov). Plan check: Full architectural drawings plus site plan; sprinkler plan required for most mid-block cottages. Pre-approved plans availability: None — Somerville does not offer a pre-approved ADU plan library.
Average ADU rental income in Somerville: 1-bedroom units rent for $2,600-$3,000/month, 2-bedroom units for $3,300-$3,700/month. Short-term rentals: Effectively no — Somerville's STR ordinance (registration required since April 2023) limits short-term rentals to the operator's primary residence, so a standalone ADU generally cannot be an Airbnb. Expected ROI: 6-9% annual return.
The minimum lot size for an ADU in Somerville is No minimum lot size for the cottage itself, but NR lots require 80 ft minimum lot depth for a backyard cottage (SZO Table 3.1.12). 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 $150,000-$250,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.