Thinking about a backyard ADU? What to know in Greater Boston

An accessory dwelling unit (ADU) — a backyard cottage, an apartment over the garage, or a finished unit attached to your house — is one of the most-asked-about projects in Greater Boston right now. For family, for rental income, or to age in place, it can be a smart use of a lot you already own.

What changed in Massachusetts

Under the state's 2024 Affordable Homes Act, since February 2, 2025 homeowners in single-family zoning districts can build one ADU "by right" — meaning no special permit — as long as it's no larger than 900 square feet or half the size of the main house, whichever is smaller. Cities and towns can still apply reasonable rules, like setbacks and site-plan review, so your local zoning still matters. Many communities, Somerville included, have their own ADU rules on top of the state's — check yours before you design.

Detached, attached or conversion?

• Detached backyard cottage — the most privacy, usually the highest cost (new foundation and utility runs).
• Attached addition — shares a wall and often shorter utility runs.
• Garage or basement conversion — often the most affordable, if the structure, ceiling height and egress work.

What drives the cost

1. Utilities. How far water, sewer and electric have to travel — and whether your electrical service needs an upgrade.
2. Foundation and site access. Tight urban lots mean smaller equipment and more hand work.
3. Size and layout. Kitchens and bathrooms are the expensive square feet.
4. Finishes. The same selections conversation as any renovation.

The first three things to check

1. Your zoning district and lot — setbacks, lot coverage and where an ADU could physically fit.
2. Your utilities — where the sewer line runs and what electrical service you have today.
3. Your goal — rental, family or home office changes the right layout.

How I help

I've mapped backyard ADU fit across Somerville parcel by parcel, and I have ready-to-adapt ADU plan sets under 900 square feet. We start with a quick feasibility check of your lot, then a 3D design and a line-item budget — before you commit to anything. Ask me about your lot.

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 →