Can I build an ADU in Medford, Massachusetts?
Yes. Medford's zoning (codified through April 2024) allows one accessory dwelling unit per lot, as of right inside an existing home or by special permit in an existing detached structure, capped at 900 sq ft or half the main home's floor area, whichever is smaller. The ordinance's owner-occupancy affidavit and special-permit requirements predate the February 2025 state ADU law, which may override them for one ADU per lot.
| Inside existing home | As of right (building commissioner) [1] |
| In existing detached structure | Special permit from the Community Development Board [1] |
| Max size | 900 sq ft or 1/2 of principal dwelling floor area, whichever is smaller [3] |
| Number per lot | 1 [3] |
| Owner occupancy | Affidavit that owner lives in one unit (may be overridden by state law) [2] |
| Minimum rental term | 30 days [5] |
| Parking | No more than 1 additional space, when necessary and feasible [6] |
What the ordinance allows
These rules come from the zoning ordinance as codified through April 16, 2024, and may have changed since. Under Sec. 94-8.2 of Medford's zoning, an accessory dwelling unit (ADU) can be created as of right inside an existing principal dwelling, or by special permit from the Community Development Board in an existing detached structure on the same lot, such as a garage or carriage house [1]. The ADU is listed as an allowed accessory use in the residence districts. Only one ADU is allowed per lot, and it can't be larger than half the main home's floor area or 900 square feet, whichever is smaller [3]. The board may make size exceptions for a detached structure with preservation value, such as a carriage house or barn.
Design and appearance
The building holding the ADU must still look like a single-family home or its accessory structure: no more than a 10% increase in gross floor area and no new entrances facing the main street [4]. You must submit a plot plan showing the building, the ADU, any septic system and parking.
Owner occupancy, rentals and parking
The ordinance requires an affidavit that the owner lives in one of the two units, and the ADU can't be sold separately [2]. If either unit is rented, the minimum rental term is 30 days, so the ADU can't be used as a short-term rental [5]. The owner must provide space for no more than one extra parking space when necessary and feasible [6]. Special permits expire after five years but can be renewed by the building commissioner with an affidavit.
State law may override parts of this
The zoning text we reviewed is codified through April 2024, before Massachusetts' ADU law took effect on February 2, 2025. State law now protects one ADU per lot by right, including new detached ADUs, and bars owner-occupancy requirements and special permits for that ADU, so state law may override the ordinance's owner-occupancy affidavit, special-permit, "existing structure only" and five-year renewal provisions. Check with the Building Department on how it applies these rules today.
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Next steps
- Call the Medford Building Department (City Hall Room 115A, 781-393-2509) to confirm how it applies Sec. 94-8.2 alongside the 2025 state ADU law.
- Prepare a plot plan (a mortgage inspection survey is enough) showing the house, the proposed ADU, any septic system and parking.
- Apply for a building permit; if the city still requires it for an ADU in a detached structure, apply to the Community Development Board for a special permit.
- Plan for leases of at least 30 days; the ADU can't be listed as a short-term rental.
Common questions
- Can I build an ADU in Medford, MA?
- Yes. Medford's zoning allows one accessory dwelling unit per lot, as of right inside an existing home or by special permit from the Community Development Board in an existing detached structure. The February 2025 state ADU law may override the special-permit requirement for one ADU per lot.
- How big can an ADU be in Medford, Massachusetts?
- No larger than 900 sq ft or half the main home's floor area, whichever is smaller. The building must still look like a single-family home, with no more than a 10% increase in gross floor area and no new entrances facing the main street.
- Does the owner have to live on site for an ADU in Medford, MA?
- The ordinance requires an affidavit that the owner lives in one of the two units, but that rule predates the February 2025 state ADU law, which may override it for one ADU per lot. Confirm with the Building Department.
- Can I Airbnb my ADU in Medford, Massachusetts?
- No. If either unit is rented, the minimum rental term is 30 days, so the ADU can't be used as a short-term rental.
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
- Sec. 94-8.2.2, Revised Ordinances Chapter 94 - Zoning (Municode, Supp. No. 18, codified through Ord. No. 889 adopted April 16, 2024), p. 1
Accessory dwelling units may be allowed as of right in an existing principal dwelling or by special permit from the community development board (the SPGA) in an existing detached structure on the same lot, provided that each of the following additional conditions are met.
- Sec. 94-8.2.3(2), Revised Ordinances Chapter 94 - Zoning (Municode, Supp. No. 18, codified through Ord. No. 889 adopted April 16, 2024), p. 1
An affidavit shall be provided stating that one of the two dwelling units shall be occupied by the owner of the property, except for bona fide temporary absence. No accessory dwelling unit shall be held in separate ownership from the principal structure/dwelling unit.
- Sec. 94-8.2.3(3)-(4), Revised Ordinances Chapter 94 - Zoning (Municode, Supp. No. 18, codified through Ord. No. 889 adopted April 16, 2024), p. 1
Not more than one accessory dwelling unit may be established on a lot. 4. The accessory dwelling unit shall not be larger in floor area than one-half the floor area of the principal dwelling, or 900 square feet, whichever is smaller.
- Sec. 94-8.2.3(5), Revised Ordinances Chapter 94 - Zoning (Municode, Supp. No. 18, codified through Ord. No. 889 adopted April 16, 2024), p. 1
"Significantly altered" shall mean no increase in gross floor area greater than ten percent shall be allowed and no additional entrances facing the principal street.
- Sec. 94-8.2.3(6), Revised Ordinances Chapter 94 - Zoning (Municode, Supp. No. 18, codified through Ord. No. 889 adopted April 16, 2024), p. 1
Where the accessory dwelling unit or the principal dwelling is occupied as a rental unit, the minimum occupancy or rental term shall be 30 days.
- Sec. 94-8.2.3(7), Revised Ordinances Chapter 94 - Zoning (Municode, Supp. No. 18, codified through Ord. No. 889 adopted April 16, 2024), p. 1
Sufficient and appropriate space for no more than one additional parking space, when necessary and feasible, shall be accommodated on-site by the owner to serve the accessory dwelling unit.