Can I build an ADU in Concord, Massachusetts?
Yes. Concord allows one accessory dwelling unit per lot, attached or in a detached accessory building, with a building permit from the Building Commissioner. It can be up to 1,000 sq ft, needs one parking space (none near transit), and must meet the main house's setbacks; there's no owner-occupancy rule.
| Approval | Building permit from the Building Commissioner (one ADU per lot) [1] |
| Attached or detached | Either: inside the house or in an existing/new detached accessory structure [1] |
| Max size | 1,000 sq ft gross floor area (or the 760 CMR 71.02 measure, whichever is greater), garage space excluded [2] |
| Lot size | Lot must meet the district's minimum lot size [3] |
| Parking | 1 space; none within 1/2 mile of a transit stop [4] |
| Setbacks | Same as the main house; Board of Appeals can reduce setbacks for a detached ADU by special permit [5] |
| Short-term use | ADU and main house can't be used as a B&B or tourist home [6] |
| Owner-occupancy | No owner-occupancy requirement in the bylaw [1] |
What the bylaw allows
Concord's Zoning Bylaw §4.2.2.2 (amended June 2025) lets the Building Commissioner issue a building permit for one accessory dwelling unit in a new or existing single-family house, or in a new or existing detached accessory structure such as a garage or barn [1]. Only one ADU is allowed per lot [1]. The bylaw does not require the owner to live on the property.
Size, lot and siting rules
- The ADU can be up to 1,000 sq ft of gross floor area, not counting garage space, or the floor area allowed under the state ADU regulation (760 CMR 71.02), whichever is greater [2].
- The lot must meet the minimum lot size for its zoning district [3]. The Zoning Board of Appeals can grant relief by special permit on a nonconforming lot that's too small or has too little frontage, and can reduce setbacks for a detached ADU [8].
- The ADU must meet the same setbacks as the main house [5], plus the district height limits and the lot's maximum floor area ratio.
- You need one parking space, or none if the lot is within half a mile of a transit stop [4].
- The home must be on Town sewer, or the septic system must handle the extra flow [11].
Other conditions
The ADU can't be split off or sold separately from the house [7]. Neither unit may be run as a bed and breakfast or tourist home [6]. No one may live in the ADU until the Building Commissioner issues a certificate of occupancy [10]. Submit dimensioned floor plans and a scaled site plan showing the ADU and parking.
State law
Table I still lists "two-family or additional dwelling unit" as needing a special permit in the residential districts [9], but the newer text of §4.2.2.2 provides for a building permit [1]. Since Feb. 2, 2025, Massachusetts law has protected one ADU per lot by right and bars towns from requiring a minimum lot size for it. State law may therefore override the lot-size condition [3] and any special-permit reading of Table I.
See how Concord compares with 8+ other Massachusetts towns on accessory dwelling units (adus) →
Next steps
- Check your district's setbacks, height limit and floor area ratio with the Building & Inspections office (141 Keyes Road, 978-318-3280).
- Confirm sewer service, or have a septic designer show the system can handle the ADU's added flow.
- Prepare dimensioned floor plans and a scaled site plan showing the ADU and its parking space, then apply for a building permit.
- If your lot is undersized or a detached ADU needs reduced setbacks, apply to the Zoning Board of Appeals for a special permit first.
- Get a certificate of occupancy from the Building Commissioner before anyone moves in.
Common questions
- Can I build an ADU in Concord, Massachusetts?
- Yes. Concord allows one accessory dwelling unit per lot, attached or in a detached accessory building, with a building permit from the Building Commissioner. There's no owner-occupancy rule in the bylaw.
- How big can an ADU be in Concord, Massachusetts?
- Up to 1,000 sq ft of gross floor area, not counting garage space, or the floor area allowed under the state ADU regulation (760 CMR 71.02), whichever is greater.
- Does an ADU need parking in Concord, Massachusetts?
- One parking space is needed, or none if the lot is within half a mile of a transit stop.
- Can I rent my ADU on Airbnb in Concord, Massachusetts?
- No. Neither the ADU nor the main house may be used as a bed and breakfast or tourist home.
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
- §4.2.2.2 Accessory dwelling unit, Zoning Bylaw (amended through Annual Town Meeting April 2026), p. 15
a building permit may be granted for one accessory dwelling unit in an existing or proposed single-family dwelling or existing or proposed detached accessory structure, provided that:
- §4.2.2.2(b), Zoning Bylaw (amended through Annual Town Meeting April 2026), p. 16
(b) The accessory dwelling unit shall occupy no more than 1,000 square feet of gross floor area, as defined under Section 1.3.12, but excluding space used for a garage or as defined in 760 CMR 71.02, whichever is greater, of the single-family dwelling or detached accessory structure;
- §4.2.2.2(a), Zoning Bylaw (amended through Annual Town Meeting April 2026), p. 16
(a) The area of the lot on which the single-family dwelling and accessory dwelling unit are located shall not be less than the required minimum lot size for the applicable Zoning District;
- §4.2.2.2(g), Zoning Bylaw (amended through Annual Town Meeting April 2026), p. 16
(g) One parking space shall be provided for the accessory dwelling unit, except no parking space shall be required for lots within a half mile radius of a transit stop as specified in M.G.L. Chapter 40A, Section 1A;
- §4.2.2.2(f), Zoning Bylaw (amended through Annual Town Meeting April 2026), p. 16
(f) The accessory dwelling unit shall meet the required setbacks for the primary structure of the applicable Zoning District and all other requirements for location on the lot applicable to the primary structure in the applicable Zoning District.
- §4.2.2.2(l), Zoning Bylaw (amended through Annual Town Meeting April 2026), p. 16
(l) The single-family dwelling or the accessory dwelling unit shall not be used for a bed and breakfast or tourist home under Section 5.3.15; and
- §4.2.2.2(i), Zoning Bylaw (amended through Annual Town Meeting April 2026), p. 16
(i) The accessory dwelling unit shall not be legally separated or sold apart from the single-family dwelling;
- §4.2.2.2 (relief by special permit), Zoning Bylaw (amended through Annual Town Meeting April 2026), p. 16
The Board may grant a Special Permit for relief for an accessory dwelling unit located on a legal nonconforming lot with less than the required minimum lot area or less than the required frontage for the applicable Zoning District, or a reduction in the required setbacks for a detached accessory dwelling unit, provided that the Board finds that the desired relief may be granted without substantial detriment to the neighborhood and without derogating from the intent and purpose of this Bylaw.
- Table I – Principal Use Regulations, §4.2.2, Zoning Bylaw (amended through Annual Town Meeting April 2026), p. 22
4.2.2 Two-family or additional dwelling unit SP SP SP SP no
- §4.2.2.2(e), Zoning Bylaw (amended through Annual Town Meeting April 2026), p. 16
(e) No use or occupancy of the accessory dwelling unit shall be allowed prior to the issuance of a certificate of occupancy by the Building Commissioner;
- §4.2.2.2(h), Zoning Bylaw (amended through Annual Town Meeting April 2026), p. 16
(h) The property is served by Town sewer or, alternatively, the on-site subsurface disposal system is adequate to accommodate any increased flows generated by the accessory dwelling unit;