An accessory dwelling unit (ADU) is a self-contained residential unit on the same lot as an existing single-family home. It has its own entrance, kitchen, bathroom, and sleeping area. ADUs are sometimes called in-law apartments, backyard cottages, or granny flats. They can now be built in every town in Maine with various municipal restrictions.
Whether you’re just asking questions or ready to build within the year, you’ll want to get familiar with planning and zoning terms so you can have the best understanding of what you need to undertake the project.
Assessor’s Parcel Number (APN): Known as the Map, Block, and Lot Number, this unique ID is used to identify your property on local tax maps.
Attached ADU: An ADU that is physically connected to the main home, such as a basement, addition, or space over a garage.
Building Code: In Maine, construction must comply with the Maine Uniform Building and Energy Code (MUBEC), which aligns with international codes.
By-Right Development: An ADU that is allowed without needing a special permit or discretionary review—provided it meets zoning and building code standards.
Deed Restrictions: Conditions or rules that are added to a deed for a house.
Detached ADU: An ADU that stands alone and is separate from the main house – like a small cottage or converted garage.
Interior ADU: A unit located entirely within the primary residence (e.g., a converted attic or basement), with a separate entrance.
Long-Term Rental: Rental of a unit for 30 days or longer. LD 2003 permits long-term rental of ADUs without owner-occupancy requirements at the state level.
Lot Coverage: The area of a property that can have buildings on it in relation to total lot area, usually expressed as a percentage. Zoning code specifies lot coverage for your lot.
Lot Size: Total square footage of your property.
Mixed-Use Zoning: Allows multiple uses on a property, such as commercial on the first floor of a building and residential above.
MUBEC (Maine Uniform Building and Energy Code): The state building and energy code. ADUs must comply with MUBEC if your municipality has adopted it.
Owner-Occupancy Requirement: A local rule that requires the property owner to live in either the ADU or the primary dwelling. (Note: Not required under Maine’s LD 2003 at the state level.)
Planning Board: A local board that reviews larger or discretionary land use projects. Most ADUs under LD 2003 are approved without Planning Board review if they meet requirements.
Primary Dwelling: The main residential structure on a lot. An ADU is considered secondary to the primary dwelling.
Separate Utility: An ADU that has independent utility connections. Some towns may require this for detached ADUs.
Setbacks: The minimum distance a building can be from front, side, or back property lines. A 4’ side setback means any building must be at least 4’ from the property side.
Shared Utility: An ADU that shares water, sewer, electric, or gas service with the primary home.
Zoning/Zoning Code: Local rules that determine what can be built on a site, like how tall a building can be and its location on a lot. All lots are assigned a zone, and each zone has different regulations.
Go to maineaduguide.com to find guidance, inspiration and helpful tools like the ADU Wizard, which uses public data to show what might be permissible on your lot, and a complete version of this glossary which also details fees and permits associated with constructing an ADU.
The Maine ADU Guide was developed by the Greater Portland Council of Governments in partnership with Maine Department of Economic & Community Development, municipalities and organizations across Maine.