Can I build an ADU in Lexington, Massachusetts?
Yes. In Lexington's single-family districts (RO and RS) a basic accessory apartment inside your house is allowed by right if it's no more than 1,000 sq ft with up to two bedrooms; a larger apartment in the house or one in a detached accessory structure needs a special permit from the Board of Appeals. The bylaw we reviewed also requires the owner to live on the property, a rule state law may now override.
| Basic apartment (inside the house) | Allowed by right in RO and RS [7] |
| Basic apartment max size | 1,000 sq ft, no more than 2 bedrooms [3] |
| Expanded apartment (inside the house) | Special permit; up to 40% of the dwelling's gross floor area [4] |
| Detached (accessory structure) apartment | Special permit; up to 1,000 sq ft [5] |
| Number per lot | 1 [1] |
| Owner occupancy (per bylaw) | Owner must live in one of the units [2] |
| Parking | 1 space per apartment [8] |
Three types of accessory apartment
These rules come from the Zoning Bylaw PDF incorporating amendments through Annual Town Meeting 2025; later amendments may have changed them. As of this review the town's ADU page says Lexington has not yet updated its bylaw for the state ADU law.
Lexington's Zoning Bylaw §6.7 allows one accessory apartment per lot [1]. It recognizes three kinds:
- Basic accessory apartment: inside the principal dwelling, no more than 1,000 sq ft of gross floor area and no more than two bedrooms [3]. Table 1 lists it as allowed by right ("Y") in the RO and RS one-family districts but not allowed ("N") in the RT two-family district [7].
- Expanded accessory apartment: inside the house, up to 40% of the dwelling's gross floor area (excluding parking areas), and only by special permit [4].
- Accessory structure apartment: in a detached accessory building such as a converted garage or carriage house, up to 1,000 sq ft, by special permit, with the special permit granting authority finding the building's appearance compatible with the house and neighbors [5]. The special permit granting authority is the Board of Appeals unless the bylaw names another board [6].
Owner occupancy and appearance
The bylaw requires an owner to live in one of the two units as a primary residence, with limited exceptions for temporary absences of under two years after written notice to the Building Commissioner [2]. Exterior stairways to upper stories must be enclosed, and where there are two front entrances one must read as the main entrance (§6.7.4).
Parking
The parking table requires one space per accessory apartment [8].
State law
Massachusetts' 2024 ADU law (in effect since February 2, 2025) protects one ADU of up to 900 sq ft (or half the main home, if smaller) by right, and bars towns from requiring owner occupancy or a special permit for it. Lexington's owner-occupancy rule, and the special permit for a detached apartment that would otherwise qualify, may be overridden by state law for a unit that meets the state size limits. The bylaw's own rules still apply to a larger apartment.
See how Lexington compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →
Next steps
- Check your zoning district on the town zoning map; RO and RS are the one-family districts.
- For an apartment inside your house of 1,000 sq ft or less, apply for a building permit with the Building Office (781-698-4530) with floor plans showing the unit's gross floor area and bedrooms.
- For a larger in-house apartment or a detached unit, ask the Zoning Office (781-698-4534) whether a Board of Appeals special permit is still required under the current bylaw and state ADU law.
- Plan one off-street parking space for the apartment.
Common questions
- Can I build an accessory apartment in Lexington, Massachusetts?
- Yes. In the single-family districts (RO and RS) a basic accessory apartment inside your house is allowed by right if it's no more than 1,000 sq ft with up to two bedrooms.
- Do I need a special permit for a larger or detached ADU in Lexington, Massachusetts?
- Yes. A larger apartment in the house or one in a detached accessory structure needs a special permit from the Board of Appeals. State ADU law may now override parts of the bylaw, so ask the Zoning Office.
- Does the owner have to live on the property for an ADU in Lexington, Massachusetts?
- The bylaw reviewed requires the owner to live in one of the units, a rule state law may now override.
- Can I rent an accessory apartment short term in Lexington, Massachusetts?
- No. Accessory apartments can't be used as short-term rentals.
Massachusetts' statewide ADU law
Since February 2, 2025, the Affordable Homes Act (St. 2024, c. 150, §8) has protected one accessory dwelling unit (ADU) per lot in any zoning district that allows single-family homes (M.G.L. c. 40A, §3). Your town cannot ban it, require a special permit or other discretionary approval, or "unreasonably restrict" building or renting it.
What counts as a protected ADU:
- A self-contained unit with its own sleeping, cooking and bathroom facilities, on the same lot as the main home.
- No larger than 900 sq ft or half the main home's floor area, whichever is smaller.
- It can be attached or detached, and it needs a separate entrance that meets the building code.
What towns cannot require:
- That the owner live in the main home or the ADU.
- More than 1 extra parking space, or any extra space if the lot is within 0.5 miles of a subway, commuter rail, ferry or bus station.
- A minimum lot size, caps on the number of ADUs, or limits on who may live there (age, income, relationship).
What towns can still do: use reasonable site plan review, setback, height and bulk rules, require Title 5 septic compliance, and restrict or ban short-term rentals of ADUs. A second ADU on the same lot needs a special permit. State regulations at 760 CMR 71.00 (in effect since January 31, 2025) explain which rules count as unreasonable.
What the bylaw says
- §6.7.3(1), Zoning Bylaw, Chapter 135 (incorporating amendments through Annual Town Meeting 2025), p. 57
1. There shall be no more than one accessory apartment on a lot.
- §6.7.3(2), Zoning Bylaw, Chapter 135 (incorporating amendments through Annual Town Meeting 2025), p. 58
occupy one or the other of the dwelling units as a primary residence, except for temporary absences as provided herein.
- §6.7.5, Zoning Bylaw, Chapter 135 (incorporating amendments through Annual Town Meeting 2025), p. 58
1. The apartment shall be located in the principal dwelling. 2. The gross floor area of the apartment shall not exceed 1,000 square feet. 3. There shall not be more than two bedrooms in the apartment.
- §6.7.6, Zoning Bylaw, Chapter 135 (incorporating amendments through Annual Town Meeting 2025), p. 58
6.7.6 Expanded Accessory Apartment. The SPGA may grant a special permit for an expanded accessory apartment if the following criteria are met: 1. The gross floor area of the apartment shall not exceed 40% of the gross floor area of the dwelling, excluding areas of the structure used for parking.
- §6.7.7, Zoning Bylaw, Chapter 135 (incorporating amendments through Annual Town Meeting 2025), p. 59
1. The gross floor area of the apartment does not exceed 1,000 square feet. 2. The SPGA determines that the exterior appearance of the accessory structure is compatible with the principal dwelling on the same lot and with dwellings and accessory structures on adjoining lots.
- §10 Definitions, Special Permit Granting Authority, Zoning Bylaw, Chapter 135 (incorporating amendments through Annual Town Meeting 2025), p. 124
The authority empowered to grant special permits, which shall be the Board of Appeals unless some other board is so designated in these bylaws.
- Table 1, Permitted Uses, A.2.02, Zoning Bylaw, Chapter 135 (incorporating amendments through Annual Town Meeting 2025), p. 129
A.2.02 Basic accessory apartment N Y Y
- §5.1.4 Table of Parking Requirements, Zoning Bylaw, Chapter 135 (incorporating amendments through Annual Town Meeting 2025), p. 18
Dwelling unit, accessory apartment, rooming unit, 1 per apartment or unit