Garage conversion ADUs

Usually. A garage conversion is a protected ADU like any other: one per lot, by right, no larger than half the main house's gross floor area or 900 sq ft, whichever is smaller. Garage parking areas don't count in the main house's gross floor area, so converting one doesn't shrink your cap, but the converted space counts as the ADU's.

The first question is whether the garage is attached. A detached garage becomes a small house built to the standard of a new single-family home. An attached garage becomes a second dwelling unit in the building, and the code then treats the whole house as a two-family.

In a two-family, the units are separated by wall and floor assemblies with at least a 1-hour fire-resistance rating, or 1/2-hour if the whole building is sprinklered (IRC R302.3). The walls run tight to the exterior walls and from the foundation to the underside of the roof sheathing. In the attic, the wall can stop at a ceiling of at least 5/8 in Type X gypsum with a draft stop, if the framing that supports it has at least 1/2 in gypsum. Anything holding up a rated floor needs an equal rating.

In practice that means opening the shared wall, the ceiling where the garage meets the house, and sometimes the attic. Smoke and CO alarms get interconnection rules of their own in an unsprinklered two-family.

Garage slabs are built for cars: usually uninsulated, often without a vapor retarder, sloped to the door, and a step or more below the house floor. Most conversions build a new floor over them, rigid insulation and a sleeper or framed floor, which deals with the slope and the cold and brings the floor up to meet the house.

Every inch raised comes out of the ceiling, and habitable rooms need at least 7 ft (6 ft 8 in for bathrooms and laundry). Some garages have the headroom to spare; some need the ceiling opened to the rafters or the slab replaced. We measure before we promise a layout.

The overhead door becomes a framed wall with windows and, often, the ADU's own entrance. That one change does most of the work of making it read as a cottage instead of a garage.

A room that newly becomes a bedroom needs an emergency escape window. In Massachusetts a double-hung can have 3.3 sq ft of net clear opening, at least 20 in by 24 in. Photoelectric smoke alarms and CO alarms go in the new unit.

Losing the bays is the other half of the decision. A town can require at most one extra parking space for the ADU, and none if any part of the lot is within half a mile of qualifying transit. Middleton's bylaw, for one, calls for a ZBA special permit to build a new garage to serve an ADU.

The walls, ceiling and floor all need insulating to today's energy code. Which path a conversion takes (prescriptive, alteration or a HERS rating) depends on its size and the town's code tier, and no source we checked settles how the ADU HERS column applies to space converted inside an existing building. We confirm the path with the building department and a HERS rater first. On septic, any new bedroom counts at 110 gallons per day and makes it New Construction under Title 5.

We measure the garage, slab and headroom first, model the conversion in 3D so you can see the old door opening become a wall of windows, and price it line by line. Then permits, the build, and a portal where you can watch it happen day by day.