Somerville allowed ADUs before the state acted: its 2019 form-based zoning overhaul legalized backyard cottages in the Neighborhood Residence (NR) district, and a September 2022 amendment made them by-right (no site plan review or neighborhood meeting). Since February 2, 2025, the state Affordable Homes Act (MGL c.40A §3, 760 CMR 71) additionally guarantees ADUs up to 900 sq ft or 50% of the principal dwelling by right wherever single-family homes are permitted, with no owner-occupancy requirement. Somerville's local backyard-cottage building type is capped at a 576 sq ft floor plate and 1.5 stories with 3 ft side/rear setbacks; a third dwelling unit on an NR lot triggers a permanent affordability requirement under the local ordinance. The city's 5-year zoning review may further revise ADU standards — check with Planning before designing.
Zoning & Size Requirements [1][2][3]
Yes — Somerville allows ADUs. A detached ADU can typically be up to 900 sq ft under current Massachusetts and Somerville zoning rules.
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| Allowed Zones | 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 |
| Max Size (Detached) | 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) |
| Max Size (Attached) | 900 sq ft or 50% of principal dwelling gross floor area, whichever is smaller (state cap); internal conversions (basement/attic) follow the same cap |
| Max Size (JADU) | No JADU category in Massachusetts law |
| Max Height | Backyard cottage: 1.5 stories max, ground story height 12 ft max, flat or gable roof only (SZO Table 3.1.12) |
| Max Units | 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 |
Setback Requirements [1][2][3]
Somerville requires a rear setback of about 3 ft for a detached ADU, and no additional parking is required. Owner-occupancy is set by local rules (confirm with the city).
| 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) |
| Parking Required | 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) |
| Owner Occupancy | Not allowed — prohibited statewide by the Affordable Homes Act effective Feb 2, 2025 |
| Min Lot Size | 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) |
Permit Process & Fees [4][5]
An ADU permit in Somerville typically costs Confirm with Inspectional Services — building permit fees are valuation-based; budget several thousand dollars on a typical $300K+ build and takes Building-permit plan review typically 4-8 weeks — Confirm with Inspectional Services (617-625-6600 ext. 5600, isd@somervillema.gov), through a standard permit review.
| Approval Process | By right: building permit through Inspectional Services (apply online via Citizenserve at somervillema.gov/citizenserve); no special permit, site plan review, or discretionary zoning approval since the 2022 amendment and the 2025 state law. Design standards apply: 3-ft paved walkway sized for an ambulance cot, buried utility services, and automatic sprinklers unless the cottage is on a corner lot, served by a driveway, or abuts an alley |
| Review Timeline | Building-permit plan review typically 4-8 weeks — Confirm with Inspectional Services (617-625-6600 ext. 5600, isd@somervillema.gov) |
| Permit Fees | Confirm with Inspectional Services — building permit fees are valuation-based; budget several thousand dollars on a typical $300K+ build |
| Impact Fees | None for a single ADU — Confirm with Inspectional Services |
| Plan Check | Full architectural drawings plus site plan; sprinkler plan required for most mid-block cottages |
| Pre-Approved Plans | None — Somerville does not offer a pre-approved ADU plan library |
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Construction Costs [6]
Building an ADU in Somerville typically costs $275,000-$450,000 ($400-$600 per sq ft); a garage conversion runs $150,000-$250,000.
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| Average Build Cost | $275,000-$450,000 |
| Cost per Sq Ft | $400-$600 |
| Garage Conversion | $150,000-$250,000 |
| Prefab ADU (Installed) | $180,000-$350,000 installed (site work, foundations, and buried utilities extra — confirm with manufacturer) |
Finance Your ADU Project
Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders.
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Rental Income Potential [7][8]
A 2-bedroom ADU in Somerville rents for around $3,300-$3,700/mo, an estimated 6-9% annual return.
| Avg Rent (1 Bedroom) | $2,600-$3,000/mo |
| Avg Rent (2 Bedroom) | $3,300-$3,700/mo |
| Short-Term Rental | 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 |
| Est. Annual ROI | 6-9% annual return |
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Recent Law Changes
What's Changed Recently
- Feb 2, 2025 — Massachusetts Affordable Homes Act §8 and 760 CMR 71 took effect: ADUs up to 900 sq ft/50% of the main home by right, no owner-occupancy, capped parking requirements
- Sept 2022 — City Council made backyard cottages by-right, eliminating site plan review, neighborhood meetings, and Urban Design Commission hearings
- Dec 2019 — Citywide form-based zoning overhaul first legalized backyard cottages/carriage houses in the NR district
- 2025-2026 — Somerville's 5-year zoning review is underway and may revise ADU/cottage standards to align local dimensional caps (576 sq ft) with the state's 900 sq ft protection
Tips for Building an ADU in Somerville
- Somerville's local cottage cap (576 sq ft floor plate, 1.5 stories) is tighter than the state's 900 sq ft protection — ask Planning which standard governs your design before paying for drawings
- Mid-block lots usually need an automatic sprinkler system in the cottage; corner lots, lots with a driveway to the cottage, or alley lots are exempt — this can swing your budget $15K+
- Adding a 3rd unit to your lot triggers Somerville's permanent affordability requirement; keeping your project as the 2nd unit on a single-family lot avoids it
- Nearly all of Somerville is within 1/2 mile of a T station, so no parking can be required — don't let anyone tell you otherwise
- Budget for buried utility service and a 3-ft ambulance-accessible walkway — both are mandatory design standards for backyard cottages
- Plan foundations for New England frost depth (4 ft) and avoid pouring in deep winter — schedule excavation April-November
ADU Builders in Somerville
Looking for an ADU builder in Somerville? ADU projects need a contractor experienced with local zoning, the permit process, and utility connections — not every general contractor has built one. We match Somerville homeowners with experienced local ADU contractors who provide free, no-obligation quotes. Getting at least three bids typically saves 20-40% on the same scope.
Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Somerville Zoning Ordinance §3.1.12 Backyard Cottage (enCodePlus) Municipal Code — Accessed 2026-07-10
- Somerville Zoning Ordinance eReader (2023 codification), pp. 73-75 — Backyard Cottage standards Municipal Code — Accessed 2026-07-10
- Accessory Dwelling Units — Mass.gov (EOHLC) state_statute — Accessed 2026-07-10
- ISD Building Division — City of Somerville city_website — Accessed 2026-07-10
- Somerville Zoning Ordinance §3.1.12(g) Design Standards Municipal Code — Accessed 2026-07-10
- How Much Does It Cost to Build an ADU in Boston? — Horizon ADU news_media — Accessed 2026-07-10
- Average Rent in Somerville, MA — Zumper Rent Research news_media — Accessed 2026-07-10
- Short-Term Rental Policy — City of Somerville city_website — Accessed 2026-07-10
Frequently Asked Questions
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