| Max size | 1100 or 50% of the primaryconfirmed with the town |
|---|---|
| Min size | 720 sq ftconfirmed with the town |
| Bedrooms | Up to 2confirmed with the town |
| Type | Attached or detachedby right |
| Parking | 1confirmed with the town |
| Owner-occupancy | Requiredin either the ADU or the primary home |
| Short-term rental | Not stated |
| Utility connection | Tie into the existing electrical connection or choose a separate metersame statewide |
The July 2025 state law (RSA 674:71–73) overrides local ordinance language that conflicts with it — even language a town still has posted. Here's what that means in Brentwood:
State law says a town may not cap your ADU below 750 sq ft — you're entitled to build up to 750 sq ft no matter what a local formula produces. Brentwood caps your ADU at whichever is smaller: 1,100 sq ft, or half the size of your main house. If your house is under about 1,500 sq ft, that 50%-of-primary rule would cap your ADU below the state's 750 sq ft floor — and for houses under roughly 1,440 sq ft, the 50% figure drops below the ordinance's own 720 sq ft minimum, an internal contradiction in Brentwood's own text. Where the formula dips below 750, state law overrides it.
Brentwood requires every ADU to be at least 720 sq ft, but the state law that created your ADU right only discusses capping the maximum — it says nothing about a town forcing you to build bigger than you want. Whether a town-imposed minimum size survives HB 577 is a genuine open question worth an attorney's read.
Under state law, you can satisfy the one-extra-space requirement at a legally dedicated off-site location instead of squeezing it onto your own lot — Brentwood's ordinance requires the space and limits you to a single driveway, but never mentions that off-site right, which you hold regardless. Separately, it isn't confirmed whether Brentwood requires parking for ordinary single-family homes in the first place — that's the legal prerequisite for the town to require ADU parking at all — so ask the town about its general residential parking standard.
CONFIRMED WITH THE TOWN (July 28, 2026) — ADUs allowed: 1; maximum size 1100 or 50% of the primary; minimum size 720 sq ft; up to 2 bedrooms; impact fees: $6,736; parking: 1; detached-unit distance rule: no; setbacks: Largest residential zone in Brentwood is Residential/Agricultural Distric - 2 acre minimum - 75" Front, 25' Side and rear. Town contact: Mark Kennedy - Land Use Administrator 603-642-6400 ext. 116.
In no case shall there be permitted more than a single-family residing within the accessory second dwelling unit. Dormitory-type facilities are expressly prohibited whether seasonal or otherwise.
The gross size of the building footprint of a detached ADU, including apparent uses, shall not exceed 1,100 square-feet. Appurtenant uses include attached garages, porches, decks, storage areas, or any other attached or interior areas accessory to the ADU.
Start with the state-law baseline: since July 1, 2025, New Hampshire law (RSA 674:71-73, as amended by HB 577) entitles you to one ADU — attached or detached — by right wherever single-family homes are permitted, and if a town caps ADU size, that cap cannot be set below 750 sq ft. Brentwood's ADU ordinance (Zoning §900.004) already matches the core mandate: one accessory dwelling unit per lot, attached or detached, 'by right' — no special exception, conditional use permit, or board hearing — and it even permits building the ADU at the same time as a brand-new principal house, which many NH towns still get wrong. Where Brentwood's text falls short of the state floor: its size formula caps the ADU at whichever is SMALLER — 1,100 sq ft, or 50% of the size of the primary house. For a typical smaller Brentwood home (under roughly 1,500 sq ft), that 50% rule would push your legal maximum below the 750 sq ft the state guarantees — and where it does, state law overrides the formula. For houses under about 1,440 sq ft, the 50% figure even falls below the ordinance's own stated 720 sq ft MINIMUM, an internal contradiction in the town's own text. Speaking of that minimum: Brentwood requires every ADU be at least 720 sq ft, but state law only discusses capping a maximum — whether a town can force a minimum size at all is a genuinely open legal question. On parking, Brentwood requires one extra off-street space and allows only a single driveway, but state law lets you place that space at a legally dedicated off-site location instead — a right you hold even though the ordinance doesn't mention it. The rest of the picture: two bedrooms max, owner-occupancy required (correctly not specifying which unit — an owner-occupancy requirement is allowed under state law), no condo conversion, septic/water certification, and an impact fee (amount not stated in the ordinance section) before occupancy. IMPORTANT CONTEXT: every dated amendment in the ADU section (3/2009, 3/2020, 3/2021, and the newest, 3/2025) predates the July 1, 2025 effective date of HB 577, so it's worth confirming with the town directly whether more recent revisions exist. See conflicts[] for full citations.
Get a free personalized report for your address in Brentwood — including where the 2025 law works in your favor.
Get My Free Brentwood ADU Report →House Bill 577 (2025) amended RSA 674:72, effective July 1, 2025. Wherever Brentwood's ordinance is stricter than this floor, the state law controls. Read the full NH ADU law explainer →
Yes — Brentwood allows one accessory dwelling unit (attached or detached) by right for owner-occupied properties. Under RSA 674:72 (as amended effective July 1, 2025), one accessory dwelling unit — attached or detached — is allowed by right on single-family lots statewide.
1100 or 50% of the primary — confirmed with the town. State law doesn't let any town cap ADU size below 750 sq ft.
Attached or detached (by right). Since July 1, 2025, state law allows detached ADUs by right statewide — town provisions that prohibited them are superseded.
1 — confirmed with the town. State law caps what any town can require at one additional space, located on- or off-site at the owner's choice.
Required — in either the ADU or the primary home. Where owner-occupancy is required, state law lets the owner choose which unit to live in.