Accessory Dwelling Units
ADU Design and Construction
An accessory dwelling unit turns extra space into an independent home—for a parent, an adult child, a guest, or future rental flexibility. We design ADUs that live well and meet the standards independent occupancy requires.
What ADU Design and Construction Includes
- Internal, attached, or detached ADUs
- Kitchen, bath, and private entrance
- Utilities and life-safety planning
- Permitting and legal occupancy
- Foundations, framing, and finishes
- Multigenerational and rental layouts
Every element is coordinated as one design-build project—see how the approach comes together in our related collection.
Our Design-Build Process
Because one accountable team handles both design and construction, your adu design and construction follows a clear path: consultation, existing-condition review and 3D scanning, design and scope development, budget alignment, permitting, construction coordination, and a final walkthrough. Learn more about our team.
Feasibility before design
Massachusetts made one ADU an allowed use by right in single-family districts, so zoning is rarely what stops a project. The constraints that matter are physical, and they are worth resolving before anyone draws a plan.
- Wastewater capacity, which on an unsewered lot is usually the deciding factor and the longest lead item
- Your actual size cap, which is 900 square feet or half the main house's floor area, whichever is smaller
- Setbacks and dimensional standards, which cannot be stricter than those applied to the main house
- Distance to transit, which determines whether any parking has to be added
- Floodplain, aquifer protection, wetlands, and historic district review where applicable
You can get a first read in about a minute with our ADU feasibility checker, or read the full guide to accessory dwelling units in Massachusetts.
Attached, detached, or a conversion
The structure type sets both the cost and the permitting path, and the cheapest option is often not the one people arrive imagining.
Converting existing space, a basement or an attic, is the least expensive route when ceiling height, egress, and systems cooperate. A garage conversion is next, since the shell already exists but insulation, conditioning, and slab work still apply. An attached addition requires new foundation and a roof tie-in. A detached building is the most flexible and the most expensive, carrying its own foundation, utility runs, and site work. We price the realistic options side by side rather than assuming the one you asked about.
Service Areas
Capstone provides adu design and construction throughout Greater Boston, MetroWest, and Norfolk County, including Boston, Foxborough, Newton, Wellesley, Weston, Needham, Brookline, Lexington, Cambridge, Belmont, Milton, Hingham, Walpole, Sharon, Mansfield, Wrentham, Norfolk, Plainville, and the surrounding communities. See all areas we serve.
Related services
All services →Frequently asked questions
What is an ADU?
An accessory dwelling unit is a self-contained residence—kitchen, bath, and typically its own entrance—created within or alongside an existing home.
Do ADUs require special permits?
Yes. ADUs are held to the standards of independent living, so permitting, utilities, and life-safety planning are central to the design.
Can a basement or garage become an ADU?
Often, yes. Basement and garage-to-ADU conversions are among the most efficient paths to a legal, comfortable second dwelling.
How big can our ADU be?
Up to 900 square feet, or one half of the principal dwelling's gross floor area, whichever is smaller. On a 1,400 square foot house that means roughly 700 square feet, not 900. Many published summaries state this backwards.
Do we need a special permit for an ADU?
Generally no. A first protected-use ADU in a single-family district is allowed by right, which means no special permit and no zoning board hearing. Narrow exceptions survive for additional ADUs on the same lot and within floodplain or aquifer protection overlays.
Can we rent the ADU out?
Long-term rental is generally permitted, and a town cannot require that either unit be owner-occupied. Short-term rental is one of the few things municipalities are still allowed to restrict, so confirm your town's rule before counting on that income.
Planning an adu design and construction project?
Book a private design-build consultation. We'll explore your goals, your home, and what's possible.
Schedule a Design Consultation →Typically a 45-minute conversation. No pressure, no obligation.