What South Shore homeowners should know before adding an accessory dwelling unit, from the state's by-right law to septic systems and builder selection.
If you own a home on the South Shore and you're considering an ADU (accessory dwelling unit) for family, rental income, or long-term flexibility, you're probably asking three questions: is it legal on my lot, what will it actually cost, and who builds these well in my area?
Short answer: Yes, you can build an ADU on the South Shore of MA. Since February 2, 2025, Massachusetts law allows ADUs under 900 square feet by right in single-family zoning districts, with no special permit required. Most South Shore projects fall between $100,000 - $360,000 depending on the type of unit, and a typical project takes five to eight months from design through construction.
Key Takeaways
- ADUs under 900 square feet (or half the main home's gross floor area, whichever is smaller) are buildable by right in single-family districts statewide.
- Towns cannot require owner-occupancy: you can build an ADU and rent it out without living on the property.
- Typical cost ranges: basement or garage conversions $100,000 - $250,000, attached units $150,000 - $325,000, detached cottages $200,000 - $360,000.
- Title 5 septic compliance still applies, which matters in many Plymouth County towns.
- A realistic timeline is five to eight months: 1 to 2 months for design, 1 to 2 months for permitting, 3 to 4 months for construction.
Can You Build an ADU on the South Shore of Massachusetts?
In most cases, yes. The Affordable Homes Act made ADUs under 900 square feet a by-right use in single-family zoning districts across Massachusetts, effective February 2, 2025. That means no special permit and no variance for a single ADU on a qualifying lot of any type. This includes a detached cottage, attached unit, or basement or garage conversion. The law also prohibits owner-occupancy requirements, so homeowners can build and rent an ADU without living in either unit, and towns may require at most one additional parking space (none if you're within a half mile of public transit). The full details are on the state's official ADU page at Mass.gov.
By right does not mean unregulated. You still need a building permit, compliance with the Massachusetts State Building Code (780 CMR) and the state ADU regulations (760 CMR 71.00), Title 5 septic compliance where applicable, and a certificate of occupancy before anyone moves in. And each town still sets its own reasonable dimensional rules, so what's buildable in Weymouth may not be identical to what's buildable in Braintree. Our guide to Massachusetts ADU regulations breaks down the statewide law and links to town-by-town guides.
Three terms that come up in every ADU conversation:
- By right: the use is allowed without a special permit, variance, or discretionary zoning approval.
- Title 5: the Massachusetts code governing septic systems; it still applies to ADUs on septic.
- Gross floor area: the floor-area measure used to apply the "half the main home or 900 square feet" size cap.
How Much Does an ADU Cost on the South Shore?
Costs depend far more on the type of unit than the town. These are the ranges we quote homeowners across our Greater Boston service area:
| ADU Type | Typical Cost Range | Best For |
|---|---|---|
| Basement or garage conversion | $100,000 - $250,000 | Working within an existing structure |
| Attached ADU (addition or converted space) | $150,000 - $325,000 | Tighter lots and connected living |
| Detached backyard cottage | $200,000 - $360,000 | Privacy, separation, and rental appeal |
On top of construction, plan for design and architecture at $3,000 - $8,000, a survey or plot plan at $3,000 - $5,000 (always required), and civil engineering at $2,000 - $4,000 where the town requires it.
ADUs typically cost more per square foot than a simple addition because they do more: an independent kitchen, bathroom, entrance, and utilities. For sizing context, studios generally run 350 to 450 square feet, one-bedrooms 450 to 650 square feet, and two-bedrooms 650 to 900 or more square feet. If financing is the open question, MassHousing offers second-mortgage loans of up to $250,000 for detached ADUs and $150,000 for attached units for eligible homeowners; our guide to ADU financing in Boston covers the options.
Town Rules Vary: Weymouth, Braintree, Milton, and more
The statewide law sets the floor, but dimensional rules like setbacks, height, and utility connections are still local. We maintain town-specific regulation guides for several South Shore area communities, including Weymouth, Braintree, and Milton. Before you commit to a floor plan, confirm your town's specific setback and height limits.
A plan that's compliant on paper still has to be buildable on your specific lot.
Septic Systems and Title 5 in Plymouth County
Many South Shore homes, particularly in Plymouth County towns, rely on septic systems rather than municipal sewer. The state's ADU law explicitly preserves Title 5 requirements, so your septic system's capacity is a real feasibility question, not a formality. Before design begins, find out whether your system can support the added bedrooms or whether an upgrade belongs in your budget. Our guide to hidden ADU costs in Massachusetts covers septic alongside the other budget items that catch homeowners off guard.
How to Choose ADU Builders on the South Shore of MA
Searching for accessory dwelling unit contractors on the South Shore mostly turns up small local builders, and many do good work. The more useful question is which builder actually specializes in ADUs, because the zoning, feasibility, and code questions above are where ADU projects succeed or stall.
A few things worth checking regardless of who you talk to: an unrestricted construction supervisor's license, a home improvement contractor registration, full insurance, and real familiarity with your town's ADU rules. We've written a full guide on how to choose an ADU builder in Massachusetts.
For our part, Horizon ADU designs and builds stick-built, on-site ADUs exclusively, both detached and attached, and we actively analyze ADU regulations across six Greater Boston counties, including Plymouth, Norfolk, and Bristol on the South Shore. Building on-site rather than in a factory lets us tailor each unit to the lot's conditions, the town's zoning, and how your family actually plans to use the space. As Paul, a homeowner in Milton, put it: "They provide quality workmanship and the team is highly responsive during both the design and construction phases. Great experience."
What's a Realistic Timeline?
A typical project runs five to eight months end to end:
- Architectural design and survey: 1 to 2 months
- Permitting: 1 to 2 months
- Construction: 3 to 4 months
Our post on how long it takes to build an ADU walks through each stage and what drives delays.
Where to Start on the South Shore
The clearest first step is finding out what your lot actually allows. You can enter your address in our ADU Planner to see your property's ADU potential, and order our $200 feasibility report there, which tells you what's buildable before you invest in design. When you're ready to talk through your property directly, contact us and we'll go from there.
The best ADU projects begin with clear feasibility, not assumptions.
Frequently Asked Questions
Do I need a special permit to build an ADU on the South Shore?
No, not in most cases. A single ADU under 900 square feet on a single-family lot is a by-right use under Massachusetts law, so no special permit or variance is required. You still need a building permit and must meet your town's dimensional and code requirements.
How much does it cost to build an ADU on the South Shore of Massachusetts?
Typically $100,000 - $360,000 depending on the type. Basement and garage conversions generally run $100,000 - $250,000, attached units $150,000 - $325,000, and detached cottages $200,000 - $360,000, plus design, survey, and any required engineering.
Can my septic system support an ADU?
It depends on your system's capacity, and Title 5 compliance is required. The state ADU law preserves Title 5 septic requirements, so homes on septic (common in Plymouth County) need a capacity check early in planning. A feasibility review is the practical way to find out before spending on design.
Do I have to live on the property to rent out my ADU?
No. Massachusetts prohibits towns from imposing owner-occupancy requirements on by-right ADUs. You can build an ADU and rent it out long-term without living in either the main home or the unit.
How long does it take to build an ADU on the South Shore?
About five to eight months. Plan on 1 to 2 months for design and survey, 1 to 2 months for permitting, and 3 to 4 months for construction.