Accessory Dwelling Units
Accessory Dwelling Units (ADUs) were legalized in Massachusetts state-wide in 2024. According to state law, ADUs under 900 square feet are permitted "by right" but local laws still apply, including design restrictions. ADUs may be a standalone building, a garage with living space above it, an addition to the primary home or a converted section of that home.
While the name may be new, ADUs have a long historic tradition. In fact, before Massachusetts communities adopted local zoning laws in the 1920s and 30s, there were essentially no restrictions to what people could build on their own property, so they built what they needed. Some families constructed small homes on their property to live in while they saved up to build a bigger house, later using the small house as a source of rental income. Other ADUs popped up as a second story on a carriage house for stable hands or attics that once housed multiple servants, accessed by a separate stairway. Whatever you call them — granny flats, in-law suites, garden apartments — ADUs can be an appropriate addition in historic districts.
This two-story carriage house in Salem, Massachusetts was built in the 1800s.
The Secretary of the Interior's Standards for Historic Building Additions instruct that additions should be subordinate, sympathetic and harmonious in scale, proportion, colors and materials to the original building. They should not attempt to imply that the new structure was built at the same time as the original structure - you should be able to identify which is the "real" historic building. If possible, you should not be able to see the ADU from the public-facing sides of the building.
The glass-and-brick addition to this Colonial Revival house (right) was designed by Cunningham/Quill architects. It's mostly obscured from the street and it uses the same colored brick and multi-lite windows of the original structure while remaining subordinate.
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Many homeowners will need to build an entirely new structure for their ADU. Most of the existing designs are very modern, but may not suit your historic district requirements or personal tastes. Architects can design a custom build, but it can save money to use an existing design. We've found a few floor plans inspired by older building styles and are always looking for more. This yellow home was designed by Backyard ADUs. Modular off-site construction helps keep costs lower, but they can still change certain architectural details to comply with historic district requirements, such as roof pitch, decorative half-timerbering, casement windows and types of siding.
Floor plans inspired by Victorian era homes
Resources for Municipalities:
Model Zoning Language for Accessory Dwelling Units
Salem Historical Commission Guidelines
Converting existing outbuildings to ADUs is a popular choice in historic districts, but they may be considered contributing historic structures and come with their own design restrictions.
This carriage house in Brookline was modified by Landmark Services. The structure was required to visually incorporate the hayloft opening and the extra-wide barn door as a nod to its original purpose.
