ADU Glossary

Planning and Zoning Terms

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.

Accessory Dwelling Unit (ADU) 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.

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.

Code Enforcement Officer (CEO) A municipal official responsible for ensuring that buildings and land uses comply with local and state regulations.

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.

Historic District A designated area with historic significance where building changes, including ADUs, must meet additional preservation requirements.

Interior ADU A unit located entirely within the primary residence (e.g., a converted attic or basement), with a separate entrance.

LD 2003 Maine’s 2022 housing law that mandates municipalities allow at least one ADU per lot with a single-family home and removes some local barriers to construction.

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.

Single-family/Multifamily Single-family zoning means one home on a property. Multifamily zoning means more than one home (duplex, triplex, apartment building.

Short-Term Rental (STR)
Rental of a unit for fewer than 30 days at a time. Municipalities may regulate STRs separately from long-term rentals.

Unpermitted ADU An ADU built without required building or zoning approvals. May need retroactive permitting or be subject to fines.

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.