Home addition cost in Massachusetts: bump-outs, dormers and second stories

The home addition cost in Massachusetts depends far more on the type of addition than on square footage alone. The best published benchmark, the 2025 Cost vs. Value report, puts a midrange 384 sq ft primary suite addition at $183,267 in Boston.

In Somerville, Cambridge and Medford, the other big factors are tying into a 1900s house, zoning, and access on tight lots. Here's how I break it down for my clients.

Home addition cost in Massachusetts: the published numbers

These are the Boston figures from Remodeling magazine's 2025 Cost vs. Value report. Both addition models sit on a crawlspace foundation.

Treat these as a starting point. Every house is different, and I give a written line-item budget after a free walkthrough.

Bump-out, rear addition, second story or dormer?

• Bump-out: a small extension, often for a kitchen or bath. Less space, but it still needs a foundation or cantilever, a roof tie-in and new exterior finishes.
• Rear addition: the classic one- or two-story box off the back. You get the most space per dollar, but you pay for a full foundation and excavation.
• Second-story addition: no new footprint, but the existing walls and foundation have to carry the load, and the roof comes off during construction. Engineering is a must.
• Dormer: turns a cramped attic into usable rooms. It's often the best value in a house with a steep roof and decent attic height.

What drives the cost of an addition

1. Foundation: full basement, crawlspace or slab. Ledge, water or an old fieldstone foundation next door all add cost.
2. Structure: beams, posts and headers where the new space opens into the old house.
3. Matching the old house: clapboard profiles, trim, window styles and roof pitch on a 1900–1930 home take skilled labor.
4. Lead-safe work: opening walls in a pre-1978 house means following lead-safe renovation practices.
5. Design and permits: architect drawings, a structural engineer and city review all happen before a shovel hits the ground.
6. Access and staging: on a narrow street near Union Square or Teele Square, there's often no room for a dumpster, a lumber drop or an excavator. Hand-carrying materials and extra trips add labor days.
7. Upgrades the new space forces: a bigger electrical service, a heating system that can't carry more rooms, or plumbing that has to be rerouted through the old house.

On most of the additions I price, these hidden items, not the cabinets or flooring, are what separate a realistic budget from a lowball one.

Somerville zoning: what you're allowed to add

Somerville's zoning lists exactly which additions each building type may have. In the Neighborhood Residence district, SZO 3.1.13 Building Components permits gable dormers, shed dormers, side wings and rear additions on cottages, detached houses, semi-detached houses and duplexes, but lists them as not permitted for the Detached Triple Decker building type. If you own a triple-decker, check zoning before anything else; my triple-decker zoning guide explains why.

• Rear additions must set in at least 1 ft from the side walls, stay within two stories and half the main building's floor plate, and can't have a steeper roof than the main house.
• Dormers can't project past the exterior wall or rise above the ridge, and flat roofs are prohibited.

Permits and city review in Somerville

The city's building permit fee schedule (effective January 1, 2025) charges $20 per $1,000 of estimated cost up to $100,000 and $21 per $1,000 above that, plus a $250 zoning plan review and a building code review of $4 per $1,000. The Engineering permits brochure says adding floor area, footprint, bedrooms or a new foundation can trigger engineering review too.

Addition vs. ADU vs. moving

• Addition: best when you need more space for your own household and love your street.
• ADU: better when the space is for a parent, adult child or tenant. See our 3D ADU designs.
• Moving: worth pricing honestly. Closing costs and a higher mortgage rate can rival the cost of building.

Need a hand?

I'm a licensed Construction Supervisor based in Davis Square, and I've planned additions across Somerville, Cambridge and Medford. Book a free walkthrough and I'll check what your lot allows, then send a written line-item budget, usually within 48 hours.

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 →