What changed
What the Massachusetts ADU law allows
The Affordable Homes Act, effective February 2, 2025, made one accessory dwelling unit per lot allowed by right in single-family zoning districts across Massachusetts. That means no special permit and no Zoning Board hearing for a protected-use ADU.
- Up to 900 sq ft or half the gross floor area of the main house, whichever is smaller.
- Towns cannot require the owner to live on site or that tenants be related.
- Towns may still apply reasonable dimensional rules, site plan review and Title 5 septic requirements.
- Minimum parking requirements are limited, especially near transit.
Every South Shore town has been updating its bylaws, so the details differ. We check yours on the first visit.
Types of ADU
Detached, attached or converted
Backyard cottage
A small separate house with its own foundation, utilities and entry. The most privacy and the highest cost.
- New water, sewer/septic and electric runs
- Needs yard space and setbacks
- $250k–$450k typical
Attached ADU
An addition with its own entrance and full kitchen, sharing a wall with the main house.
- Shares utilities, lowering cost
- Can double as an in-law suite
- $200k–$380k typical
Garage apartment
Living space built over a new or existing garage.
- Existing garage must carry the load
- Fire separation required
- $180k–$350k typical
Basement or garage conversion
Existing space turned into a legal unit. The lowest cost per square foot when the space qualifies.
- Egress, ceiling height and moisture decide feasibility
- Fastest permit path
- $120k–$220k typical
Feasibility
What decides whether an ADU works on your lot
Septic capacity
An ADU adds bedrooms. On Title 5 septic, the system may need to be upgraded or expanded.
Setbacks and lot coverage
Towns can still apply dimensional rules to a detached unit.
Utility routing
Distance to water, sewer and electric service drives detached ADU costs.
Separate systems
Its own heating and cooling (usually a heat pump) and often a separate electric meter.
Wetlands and flood zones
Conservation Commission review still applies near resource areas.
Rental plans
Short-term rental of ADUs can still be restricted by your town.
Timeline
From idea to move-in
-
1
Week 1–2
Feasibility visit
Zoning, septic, utilities and where the unit can go.
-
2
4–8 weeks
Design and estimate
Floor plan, elevations and an itemized estimate.
-
3
4–10 weeks
Permits
Building permit, Board of Health if on septic, and any site plan review.
-
4
4–7 months
Construction
Foundation, framing, utilities, finishes and final inspection.
Cost
What an ADU costs on the South Shore
What moves the price
- Utility trenching distance
- Septic upgrade
- Foundation type
- Full kitchen vs. kitchenette
- Finish level
*Size limit and by-right status per M.G.L. c.40A §3 as amended in 2024. Your town bylaw details apply. Planning ranges only.
Our work
Accessory Dwelling Units projects on the South Shore
FAQ
Accessory Dwelling Units: questions homeowners ask
Do I need a special permit to build an ADU in Massachusetts?
Not for a protected-use ADU in a single-family zoning district since February 2, 2025. You still need a building permit and must meet your town’s reasonable dimensional and site rules.
How big can an ADU be?
Up to 900 sq ft or half the gross floor area of the main house, whichever is smaller. Towns may allow larger units by special permit.
Can I rent out my ADU?
Yes, long-term rental is allowed and towns cannot require the tenant to be related. Short-term rentals may still be restricted locally.
Do I have to live on the property?
The state law prohibits towns from requiring owner occupancy for protected-use ADUs.
What is the cheapest type of ADU?
Converting existing space, like a dry basement or a garage, is usually the cheapest, provided it can meet egress, height and moisture requirements.
Service area
Accessory Dwelling Units in Randolph and across the South Shore
Based at 42 Edwin Street in Randolph, we build accessory dwelling units in these towns and know how each building department works.
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