Somerville's triple-decker comeback: what the zoning allows — and why the math decides

The triple-decker built Somerville. For decades, zoning made it almost impossible to build a new one. That changed in late 2023, when the City made the Detached Triple Decker a by-right building type in its Neighborhood Residence (NR) district. So I modeled what that means on every single- and two-family lot in the city.

What the zoning allows

Under the Somerville Zoning Ordinance (§3.1.11), a new detached triple-decker in NR is roughly:

• 24–30 ft wide and 36–50 ft deep, three stories with a flat roof;
• a front setback of 10–20 ft, 5 ft side setbacks (3 ft with a front driveway) and a 20 ft rear setback;
• on a lot at least 34 ft wide (36 ft with a front driveway), covering no more than 60% of the lot;
• with no minimum parking requirement for the homes.

Older buildings come with a step first. Somerville reviews the demolition of any building 75 years or older: the Historic Preservation Commission holds a "significance" hearing within about 65 days, and if a house is found "preferably preserved," demolition can be delayed up to 18 months. Plan on roughly 3–4 months of review even in the easy case. Siding, roofs, windows, interior work, dormers and porches are not "demolition."

Sources: SZO §3.1.11 Detached Triple Decker · Somerville Code §7-28 demolition review. Always check the current text before planning around these numbers.

What we found

• We tested 7,085 single- and two-family lots.
• On paper, a by-right triple-decker fits on about 4,250 of them.
• But when you add the purchase price, demolition, a construction cost of about $295 per sq ft and Somerville resale prices, only about 330 lots clear a healthy 15% margin.

Zoning isn't the constraint — economics is. For most lots, the land plus the existing house simply costs too much for three new homes to pencil out.

What that means for owners

1. If you own a small, older house on a wide lot, you may be sitting on one of the few sites where it works. It's worth a real feasibility check.
2. If you're buying to build, the most important number is your maximum offer — what you can pay and still make the project work.
3. If the math doesn't work, a renovation, an addition or a backyard cottage often does. See what our ADU map found.

Thinking about a Somerville lot? Send me the address — I'll run it through the same model and tell you what it supports.

About the author

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.

More about Mateus →