The ADU Process, Stage by Stage

We walk the property with you and look at the things that decide whether an ADU works here at all: where the unit could sit, how equipment and materials would reach it, where water, sewer or septic, and electrical service are now, and how far they would have to travel. We talk through what you want the unit for — family, rental income, or aging in place — because that changes the layout. The visit is free and carries no obligation.

We settle on detached, attached, or an internal conversion, and on a footprint that fits both your lot and the size cap. Massachusetts protects one ADU per single-family lot at up to half the gross floor area of the principal dwelling or 900 square feet, whichever is smaller — so on a 1,400 sq ft house the by-right ceiling is 700 sq ft, not 900. Once the scope is real, so is the estimate: we replace the per-square-foot ballpark with a written number for your project.

An architect or designer produces the drawing set the building department will review — floor plans, elevations, sections, structural details, and the energy compliance path. On a septic lot this is also when the system gets evaluated, since MassDEP's Title 5 guidance governs whether the existing system can serve a second dwelling or needs an upgrade. These fees are billed by the design professionals directly, not folded into our construction price.

Under 760 CMR 71.03(1), a town cannot require a special permit, variance, or other discretionary zoning approval for a Protected Use ADU in a single-family residential zoning district. Two narrow exceptions exist: a second ADU on the same lot does need a special permit, and a town may require one in a floodplain or aquifer protection overlay if it requires one for the main house, on clear and objective criteria. Your town may still run site plan review before issuing the building permit, but the regulation requires that review to be clear and objective and consistent with an as-of-right process. Historic districts are the one place a town may apply stricter design and dimensional standards than it would to a house.

We file the building permit application with your town's building department. Under 780 CMR R105.3.1 the building official must issue or deny the permit in writing within 30 days of filing. In practice most applications come back with comments before they come back approved, and answering those quickly is most of what keeps this phase short. We handle the filing and the back-and-forth.

We stake the building, strip and excavate, and open the trenches for the utility runs. This is where surprises live — ledge, a high water table, or poor soil all change the work. A town cannot require you to install a separate utility connection, but the utility company itself, state law, or a local board still can, so we confirm early what your service actually needs.

Forms, reinforcing steel, and concrete. We build ICF foundations, which cost more than a poured wall up front and pay it back in insulation and durability. Two code inspections gate this phase: the footing and foundation inspection under 780 CMR R110.3.1, made after the excavation and any reinforcing steel are in place, and the slab and under-floor inspection under R110.3.2, made before concrete goes down over building service equipment. Nothing gets covered until the inspector has seen it.

Floor deck, walls, roof structure, sheathing, windows and doors, and the roof covering. Getting the building weather-tight is the milestone that matters here, because everything after it happens inside regardless of the forecast.

Wiring, supply and waste piping, ductwork or line sets, and ventilation, all run inside open walls. We coordinate these trades in-house so they are not tripping over each other. The frame inspection under 780 CMR R110.3.4 comes after the framing, fire-blocking, and bracing are in place and the rough mechanical, electrical, and plumbing work has been approved — so the rough-ins have to pass before the frame does.

Insulation and air sealing go in, and the building envelope gets its energy efficiency inspection under 780 CMR R110.3.7, which covers envelope and duct insulation values, window performance, and the HVAC and water-heating equipment. If your town has adopted the Stretch Energy Code or the Specialized Opt-in Code under 225 CMR 22.00, the unit also has to be verified by a third-party rater — an extra party to schedule and a higher bar to hit. Our town lookup shows which tier applies to you.

Drywall goes up and gets its lath and gypsum board inspection under 780 CMR R110.3.5 before any finishing. Then flooring, trim, cabinetry, counters, tile, fixtures, and paint. This is the phase you control most directly: selections made early keep the schedule moving, and selections made late are the single most common reason a finish phase runs long.

The final inspection under 780 CMR R110.3.10 is made after the permitted work is complete and before occupancy, and the certificate of occupancy is what legally makes the unit habitable. Your town assigns the ADU its own address under 760 CMR 71.03(8) and records the permit and certificate for state reporting. We walk the finished unit with you, build the punch list, and close it out.