Yes — Somerville homeowners have two by-right routes in 2026. The local route is the Backyard Cottage under Zoning Ordinance §3.1.12: a detached accessory building up to 24 ft wide by 32 ft deep (576 sq ft floor plate), 1.5 stories, 3 ft side and rear setbacks, one dwelling unit, allowed by right in the Neighborhood Residence (NR) district, with a mandatory ambulance-cot walkway, buried utilities and sprinklers on most mid-block lots. The state route is the Protected Use ADU under M.G.L. c. 40A §3 and 760 CMR 71 (in force since Feb 2, 2025): one attached, internal or detached unit of up to 900 sq ft or half the house's gross floor area, by right, no owner-occupancy, no parking, on any NR or UR lot whether the house is single-, two- or three-family. Somerville has not yet written the state rule into its ordinance — the Mayor's PADU Compliance Amendment (Council item 26-1336, filed Sept 2, 2026) is in the Land Use Committee — so ISD Zoning applies the state law directly; ask isdzoning@somervillema.gov which standards govern your design before paying for drawings. Building permits go through ISD on Citizenserve and no impact fees apply.
Zoning & Size Requirements [1][2][3][4][5][6][7]
Yes — Somerville allows ADUs. A detached ADU can typically be up to 576 sq ft under current Massachusetts and Somerville zoning rules.
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| Allowed Zones | 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)). |
| Max Size (Detached) | 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. |
| Max Size (Attached) | State cap only: 900 sq ft or 50% of the principal dwelling's gross floor area, whichever is smaller (760 CMR 71.02), for an attached or internal unit (basement, attic, addition). The city ordinance has no attached-ADU type today; the pending Sept 2026 amendment (§3.1.6.d) would codify internal ADUs by right where they meet 760 CMR 71 and create no new nonconformity. |
| Max Size (JADU) | 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. |
| Max Height | 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. |
| Max Units | One cottage per lot (one principal building plus one accessory building, SZO §3.1.6.a) with at most one dwelling unit in it (§3.1.12.d); the state protects one ADU per lot (760 CMR 71.02) and it is not counted toward density (71.03(2)(d)). No affordability requirement attaches to a cottage or ADU unit under the current NR text; income restrictions on a Protected Use ADU are prohibited (71.03(2)(c)). |
Setback Requirements [1][2][3][4][5][6][7]
Somerville requires a rear setback of about 3 ft for a detached ADU, and no additional parking is required. Owner-occupancy is not required — you can rent it out without living on the property.
| 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. |
| Parking Required | 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)). |
| Owner Occupancy | 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.) |
| Min Lot Size | 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). |
Permit Process & Fees [8][9][10]
An ADU permit in Somerville goes through a standard permit review. Review time: 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. Permit fees: 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.
| Approval Process | By right: apply for a building permit with the Inspectional Services Department (ISD) Building Division on the Citizenserve portal (somervillema.gov/citizenserve); no special permit, site plan approval or hearing for a Backyard Cottage or a state Protected Use ADU. An owner living in a 1- or 2-family house may pull the permit; 3-family or larger needs a licensed contractor (CSL, plus HIC for owner-occupied 1-4 unit buildings). Submit an Estimated Cost Affidavit, plans, Workplace Safety and Wage Theft affidavits (contractors also insurance, signed contract, Site Safety Plan, OSHA card). Zoning questions: isdzoning@somervillema.gov; ISD 617-625-6600 x5600, 1 Franey Rd. |
| Review Timeline | 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. |
| Permit Fees | 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. |
| Impact Fees | None. No impact or capacity fee appears in the Zoning Ordinance for a single accessory unit, and the state bars any impact fee or study on a Protected Use ADU that is not required for a single-family house (760 CMR 71.03(3)(b)6). Water/sewer connection charges: confirm with the Engineering Division. |
| 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 | 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). |
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Construction Costs [11][12][13]
Building an ADU in Somerville typically costs $220,000-$400,000 ($275-$450 per sq ft); a garage conversion runs $100,000-$250,000.
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| Average Build Cost | $220,000-$400,000 |
| Cost per Sq Ft | $275-$450 |
| Garage Conversion | $100,000-$250,000 |
| Prefab ADU (Installed) | $250,000-$480,000 |
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Rental Income Potential [14][15][16]
A 2-bedroom ADU in Somerville rents for around $3,450-$3,750/mo, an estimated 9-14% gross rent yield (estimate).
| Avg Rent (1 Bedroom) | $2,900-$3,150/mo |
| Avg Rent (2 Bedroom) | $3,450-$3,750/mo |
| Short-Term Rental | No for a rented ADU. |
| Est. Annual ROI | 9-14% gross rent yield (estimate) |
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Recent Law Changes
What's Changed Recently
- Sept 20, 2026 — Entry re-verified against the Somerville Zoning Ordinance on enCodePlus (§3.1 NR incl. §3.1.12), 760 CMR 71.02-71.03, City Council Legistar records and the ISD and STR pages: front setback corrected to 60 ft (was 32-34 ft, which are the lot-width minimums), the unsourced 80 ft lot-depth and third-unit affordability claims removed, fees and timeline marked not published, rents updated to HUD FY2027 SAFMRs.
- Sept 2, 2026 — Mayor filed the 'PADU Compliance Amendment' (Council item 26-1336, referred to the Land Use Committee Sept 10) to align §§3.1.3, 3.1.6, 3.1.8-3.1.13, 3.1.16 and 3.2.6 with the state ADU law: internal ADUs and garage/outbuilding conversions by right, cottage front setback 60 ft to 10 ft, 576 sq ft floor plate replaced by a 1,750 sq ft total GFA cap with height and massing per the principal building. Not adopted as of Sept 20, 2026.
- June 25, 2026 — Ord. 2026-05 amended Table 3.1.13 Building Components (cottages may have an entry canopy, stoop, portico, projecting porch or cross gable; no dormers, bays, side wings or rear additions).
- Feb-May 2026 — Council item 26-0234 (Feb 9) and two resident petitions (26-0329, 26-0330, Mar 6) to rewrite §3.1.12 Backyard Cottage were kept in the Land Use Committee (Mar 19, Apr 16, May 21); Council orders 26-0068 (Jan 8) and 26-0100 (Jan 22) asked the Administration to clarify ADU rules and discuss an owner-occupant incentive pilot.
- Feb 2, 2025 — Massachusetts Ch. 150 of 2024 §8 (M.G.L. c. 40A §3) and 760 CMR 71.00 took effect: one Protected Use ADU up to 900 sq ft or 50% of the house by right, no owner-occupancy, no parking within 0.5 mile of transit, no ADU-only impact fees.
- Nov 21, 2023 — Ord. 2023-23 re-adopted the NR building-type standards including §3.1.12 Backyard Cottage (576 sq ft floor plate, 24 x 32 ft, 1.5 stories) as a by-right accessory building type.
Tips for Building an ADU in Somerville
- Two routes, one lot: the local cottage (§3.1.12) is capped at a 576 sq ft floor plate, 1.5 stories and a 60 ft front setback; a state Protected Use ADU may be up to 900 sq ft or half the house's gross floor area and can be a basement, attic or addition. Until the pending amendment passes, email isdzoning@somervillema.gov with your lot and ask which standards ISD will apply.
- Size math for the state route: a 1,600 sq ft two-family gets an ADU of up to 800 sq ft (50%); anything over 1,800 sq ft is capped at 900 sq ft. Gross floor area is the measure in 760 CMR 71.02, so a finished basement with code-height ceilings counts toward the house's total.
- Two- and three-family owners qualify for the state route: 760 CMR 71.02 defines the principal dwelling as any structure with at least one dwelling unit on a lot in a district where single-family homes are allowed, which NR and UR are.
- Mid-block cottages need an automatic sprinkler system by order of the Fire Prevention Bureau; corner lots, lots with a driveway to the cottage and alley lots are exempt (§3.1.12.f.iii). Budget for it and for burying the utility service and paving a 3 ft ambulance-cot walkway.
- Check lot width first: a cottage needs a 32 ft wide lot (34 ft with a front driveway) and must sit 60 ft back from the front lot line and 10 ft from the house, 3 ft from side and rear lines (§3.1.12.a-b).
- No parking is required anywhere in NR for residential or accessory uses (Table 3.1.18), and the state bars ADU parking within 0.5 mile of any MBTA subway or bus stop — Somerville has no residential parking minimum to satisfy.
- Short-term rental is off the table for an ADU you do not live in: the STR ordinance covers stays under 28 days, requires the unit to be the operator's primary residence and bars adjacent units; register on Citizenserve if you ever host in your own unit.
- Rents by ZIP (HUD FY2027 Small Area FMR): 02143 $3,040 1BR / $3,630 2BR; 02144 $3,140 / $3,750; 02145 $2,900 / $3,470. A $300,000 cottage renting at $3,000 a month grosses about 12% a year before costs.
- Cost basis: Massachusetts industry guides dated 2026 (Dwelly Mar 2026: $275-$450 per sq ft, 600-800 sq ft $165K-$360K; Horizon ADU Jul 2026: detached 900 sq ft $250K-$360K, basement/garage conversion $100K-$250K; ADU Wizard 2026: prefab installed 650-750 sq ft $250K-$480K). Somerville sprinkler, utility-burial and walkway rules push a cottage toward the top of the range.
- Watch the PADU Compliance Amendment (item 26-1336): if adopted as filed, cottages could be as tall as the house with dormers and a 1,750 sq ft GFA cap, and garages could convert by right. Check the Land Use Committee agenda before finalizing a design that only just fits today's 576 sq ft box.
- Cost basis (2026-09-20 re-verification): short term allowed: No for a rented ADU. Somerville's short-term rental ordinance (in force Jan 1, 2020; registration required since Apr 1, 2023) allows stays under 28 consecutive days only in the operator's own primary residence and expressly bars renting an adjacent unit in the same building; unhosted stays are capped at 90 days a year. State law lets cities restrict STR of Protected Use ADUs (760 CMR 71.03(3)(c)).
ADU Builders in Somerville
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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 Neighborhood Residence (NR): §3.1.6 Building Types, §3.1.12 Backyard Cottage, Table 3.1.13, §3.1.18 Parking & Mobility (enCodePlus, current through Ord. 2026-05) Municipal Code — Accessed 2026-09-20
- Somerville Zoning Ordinance §3.1.12 Backyard Cottage (Ord. 2023-23, 11/21/2023) Municipal Code — Accessed 2026-09-20
- City Council item 26-1336, PADU Compliance Amendment (Mayor's request, filed Sept 2, 2026; referred Sept 10, 2026) — redlined text ordinance — Accessed 2026-09-20
- City Council item 26-0234, Amending Sections 2.1.1 and 3.1.12 regarding Accessory Dwelling Units (Feb 2026; kept in committee) ordinance — Accessed 2026-09-20
- 760 CMR 71.02 Definitions (Accessory Dwelling Unit, Principal Dwelling, Protected Use ADU, Single-family Residential Zoning District, Transit Station) state_statute — Accessed 2026-09-20
- 760 CMR 71.03 Regulation of Protected Use ADUs in Single-family Residential Zoning Districts state_statute — Accessed 2026-09-19
- Zoning Information — City of Somerville (ISD Zoning team contact) city_website — Accessed 2026-09-20
- ISD Building Division — City of Somerville (permit process, documents, plan-review fee, contacts) city_website — Accessed 2026-09-20
- Somerville Zoning Ordinance §3.1.12.f Design Standards (walkway, buried utilities, sprinklers) Municipal Code — Accessed 2026-09-20
- Somerville Zoning Ordinance §12.1 Affordable Housing (applicability; no requirement attaches to a single accessory unit) Municipal Code — Accessed 2026-09-20
- Dwelly, How Much Does an ADU Cost in Massachusetts in 2026? (Mar 7, 2026) industry — Accessed 2026-09-20
- Horizon ADU, How Much Does a 900 Sq Ft ADU Cost in Massachusetts? (Jul 24, 2026) industry — Accessed 2026-09-20
- ADU Wizard, How Much Does an ADU Cost in Massachusetts in 2026? industry — Accessed 2026-09-20
- HUD FY2027 Small Area Fair Market Rent Schedule (Somerville ZIPs 02143, 02144, 02145) Government Data — Accessed 2026-09-20
- Short-Term Rental Policy — City of Somerville (ordinance in force Jan 1, 2020; registration since Apr 1, 2023) city_website — Accessed 2026-09-20
- Short-Term Rentals Ordinance — City of Somerville (PDF) ordinance — Accessed 2026-09-20
Frequently Asked Questions
Can I build an ADU in Somerville?
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. 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.. Owner-occupancy is not required, so you can rent out the ADU.
How much does an ADU cost in Somerville?
ADU construction in Somerville typically costs $220,000-$400,000. Garage conversions run $100,000-$250,000, and prefab ADUs cost $250,000-$480,000.
How much can I rent an ADU for in Somerville?
Average ADU rents in Somerville are $2,900-$3,150/mo for a 1-bedroom and $3,450-$3,750/mo for a 2-bedroom unit.
Do I need parking for an ADU in Somerville?
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)).. Check with Somerville planning department for the latest Massachusetts ADU parking rules, as many states have relaxed requirements in recent years.
How long does it take to get an ADU permit in Somerville?
Somerville does not publish a standard ADU review timeline. 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. Permit fees: 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.
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