Can your Somerville backyard fit a cottage? What our lot-by-lot map found
We screened 9,332 Somerville lots with real parcel and building data. About 2,664 backyards can fit a detached ADU — and the numbers on what one is worth are eye-opening.
Mateus MagnoLicensed Construction Supervisor · October 19, 2026 · 3 min read
"Could I build a cottage in my backyard?" is one of the most common questions I get in Somerville. Instead of guessing lot by lot, I built a map that answers it for the whole city using the state's parcel boundaries and building footprints. Here's what it found.
What we checked
For every residential lot, the map draws the house that's already there and then looks for room for a well-proportioned cottage in the backyard — not squeezed into a side strip. It applies conservative rules:
- 3 ft from the property lines and 10 ft from the existing house;
- behind the house, and at least 20 ft back from the street;
- no larger than 900 sq ft, or half the size of the main house, whichever is smaller;
- overall lot coverage of 60% or less.
What it found
- 9,332 residential lots screened.
- A detached cottage fits in about 2,664 Somerville backyards under those rules.
- On another 867 lots, the realistic route is rebuilding an existing garage as a small home.
- Because so much of Somerville is close to the T, a large share of these lots fall within half a mile of transit — where the state ADU law limits parking requirements.
What a cottage is worth
Using our 20-year Somerville price analysis, here's the math on a 900-square-foot cottage:
- Build cost: about 900 sq ft × $450 = $405,000.
- Value at Somerville new-construction prices (about $851/sq ft): roughly $765,900 — about $360,000 more than it costs.
- Rent: around $3,300 a month for a comparable unit, or roughly $25,000 a year after expenses.
One honest caveat: an ADU usually can't be sold on its own unless the property is converted to condominiums. The value shows up in the whole property — and in the rent.
The rules: Somerville's cottage rules meet the state ADU law
Two sets of rules apply, and they're being brought together right now:
- Somerville already allows a "Backyard Cottage" by right in its Neighborhood Residence district (SZO §3.1.12): one home, up to about 24 × 32 ft (a 576 sq ft footprint), 1½ stories, 3 ft from the side and rear lot lines, 10 ft from the main house and at least 60 ft back from the front lot line — plus a paved path wide enough for emergency crews and, on most lots, sprinklers. NR has no residential parking minimum.
- State law protects one ADU per lot of up to 900 sq ft (or half the main home, whichever is smaller) wherever single-family homes are allowed — including lots with two- and three-family houses. Cities can't require a special permit, can't require the owner to live on site, and can't require parking within half a mile of transit.
- Somerville is updating its zoning to match the state law this fall, so the local size limits may change. Check the current ordinance — or ask me — before you design.
Sources: Somerville Zoning Ordinance §3.1.12 · Mass.gov ADU FAQs · M.G.L. c. 40A §3. A map is a screening tool, not a permit: every lot still needs a site check, a design and the City's review. For the general picture, see our ADU guide for Greater Boston.
Financing
MassHousing offers an ADU loan of up to $250,000 for a detached cottage on a single-family primary residence — you'll need plans and permits first. See our guide to rebates and loans.
Want to know if your lot made the list? Send me your address — I'll check it on the map and tell you honestly what would fit.
Mateus Magno is the founder of Magno & Co — a licensed Massachusetts Construction Supervisor (CS-123658), registered Home Improvement Contractor (HIC #220484) and certified Lead-Safe Renovator Supervisor who has renovated and managed over $3 million in projects across Greater Boston.
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