Detached ADU cottages in the back garden

In most of Massachusetts, yes. A town can't require an ADU to be attached, can't require a minimum lot size, and must permit a detached protected ADU without a special permit if it meets the definition. What the town can do is apply setbacks, height and lot coverage, as long as they're no stricter than those for the main house, a single-family home or an accessory structure in that district, whichever is most permissive.

Local bylaws add placement rules. Topsfield keeps the ADU out of the required front yard, Middleton and Boxford don't let it sit closer to the front lot line than the house, Hamilton asks for a special permit to put one in the front yard, and Salem's detached units sit at least 5 ft from side and rear lot lines and 10 ft from other buildings. We check the bylaw and the lot's conservation, flood and historic overlays before we draw anything.

The best cottages are placed, not dropped. We look at sun across the lot, which windows of the main house look at it, where the path runs, mature trees and their roots, and the grade. On Cape Ann and in Marblehead the ledge often makes the decision: a cottage that steps with the rock can mean less blasting. Within 100 ft of a wetland, the conservation commission reviews the work first.

In the 3D model you can move the cottage and watch the shadows and sightlines change before a surveyor stakes anything.

A detached ADU is built under the Residential Code to the standard of a new single-family home. That means a real foundation designed for frost: a full basement, a crawl space or a frost-protected slab, depending on the site, the water table and what you want to store. Several towns, Wenham, Hamilton and Manchester among them, write a foundation requirement into their bylaws, ruling out trailers.

Modular and prefab units are allowed. A town can't restrict a modular ADU more than the building code does, and EOHLC says banning foundation types the code permits would be unreasonable. Modular saves some site time; it doesn't remove the foundation, the crane day or the utility work.

Water, power, communications, and sewer or septic all have to reach the cottage, usually in one shared trench from the main house. A town can't require separate water, sewer or electric connections, though the utility itself still can.

On septic, this is the first question. An ADU in a new building that needs an occupancy permit is New Construction under Title 5. The design flow is 110 gallons per day per bedroom, the minimum tank is 1,500 gallons even for one bedroom, and a system shared with the house needs a two-compartment tank or two tanks in series. Hamilton, Wenham, Boxford, Topsfield, Middleton, Lynnfield, Georgetown, Rowley and West Newbury are septic towns, as are parts of Essex, Ipswich, Manchester-by-the-Sea, Gloucester, Rockport, Newbury, Andover and North Andover, and several of their boards of health have rules stricter than Title 5. We get the septic evaluated before design.

Its exterior walls don't need a fire rating because it shares the lot, but everything else a new home needs applies: photoelectric smoke and CO alarms, escape windows in bedrooms and 7 ft ceilings in habitable rooms.

For very small cottages, Appendix AQ of the 10th Edition covers dwellings of 400 sq ft or less (lofts excluded): 6 ft 8 in ceilings in habitable space, lofts of at least 35 sq ft and 5 ft in every horizontal direction, and only one egress door. Its separate energy path works only in Base-code towns. In Stretch towns, Table R406.5 sets ADU HERS limits of 52 mixed-fuel, 55 with solar or all-electric, and 58 with both.

Feasibility first: setbacks, overlays, septic and the town's bylaw. Then the cottage modeled on your lot in 3D and rendered, a line-item proposal with allowances called out, and permits pulled by us as the registered contractor. A new ADU is also assigned its own address under the MassGIS standard.