| Max size | 750 sq ft or 35% of homewhichever is greater |
|---|---|
| Min size | No town minimum'An ADU may be less than 750 SF' |
| Bedrooms | Up to 2 |
| Type | Attached or detacheddetached exempt from 1-home-per-lot |
| Parking | Required on the lotno ADU count stated; state caps at 1 |
| 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 Deerfield:
State law gives you the choice of on-site or a legally dedicated off-site spot for any required ADU parking space. Deerfield's ordinance still requires all ADU parking to be on your own lot, but state law overrides that — the town can't force it onto your lot only.
State law caps what a town can require at ONE additional parking space for an ADU. If Deerfield's building/planning office applies the general 2-spaces-per-dwelling-unit table to your ADU, that's double the cap — state law overrides it, and you cannot be required to provide more than one extra space beyond what the principal home already requires.
State law entitles you to add a detached ADU on the same lot as your house without extra acreage, the same as an attached one. Deerfield's own carve-out from extra lot-area requirements names only ATTACHED ADUs — so if the town tries to apply its 3-acres-per-dwelling-unit rule to a DETACHED ADU and demand 6 total acres, that's exactly the kind of acreage gate state law no longer allows and overrides.
State law entitles you to convert an existing nonconforming structure — like an old garage that already sits inside the current setback lines — into your ADU without relocating it. Deerfield's text as written would block that conversion, but state law overrides it.
State law bars towns from limiting an ADU's choice of ingress/egress. Deerfield doesn't just offer a shared-door option (§331.2.C already does that) — §331.2.H appears to separately REQUIRE a physical interior connecting door to exist between the house and an attached ADU, even if the homeowner would rather build fully independent access with no interior connection at all. Whether mandating that door crosses into 'limiting ingress/egress choice' under state law is a legal judgment call. It's also logically inapplicable to a fully detached ADU with no shared wall — an internal inconsistency in the town's own text.
Deerfield's ADU rules (Zoning Ordinance §331, cover-dated 2020) allow one accessory dwelling unit — attached or detached — by right in the AR Agricultural-Residential district, which is the town's only base zoning district. ADUs are a listed 'Permitted Use,' not a special exception, and §304 specifically exempts a detached ADU from Deerfield's one-dwelling-per-lot rule. Size is generous: the ADU may be up to the GREATER of 750 sq ft or 35% of the primary home's living space (no separate 950 sq ft ceiling is stated), and it may also be smaller than 750 sq ft — so the town's size rule already meets or exceeds the state-law floor. Up to two bedrooms are allowed, and owner-occupancy is required in either unit (town doesn't specify which — compliant). Where Deerfield's posted rules likely fall short of the July 1, 2025 state law floor (RSA 674:71-73 as amended by HB 577) — which overrides the town text wherever the two conflict, since no update to §331 for that law has been located: (1) the ordinance requires ADU parking to be available 'on the lot' with no off-site option — state law gives homeowners that choice; (2) reading the ADU parking clause together with the town's general 2-spaces-per-dwelling-unit table raises a real risk the town could demand 2 spaces for the ADU (4 total) — double the one extra space state law caps towns at; (3) the town's lot-area exemption from extra acreage names only ATTACHED ADUs, leaving open a 3-acre-per-unit gate for DETACHED ADUs (6 acres total) that state law doesn't allow; (4) the ordinance requires full setback compliance for an ADU with no exemption for converting an existing nonconforming structure like an old detached garage, which state law now protects; (5) a mandatory interior connecting door for attached units may cross the line into dictating ingress/egress choice — an attorney call. See conflicts[] for full citations. Until the town updates §331, the state-law floor controls wherever the two disagree.
Get a free personalized report for your address in Deerfield — including where the 2025 law works in your favor.
Get My Free Deerfield ADU Report →House Bill 577 (2025) amended RSA 674:72, effective July 1, 2025. Wherever Deerfield's ordinance is stricter than this floor, the state law controls. Read the full NH ADU law explainer →
Yes — Deerfield allows one accessory dwelling unit (attached or detached) by right in its single agricultural-residential district. 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.
750 sq ft or 35% of home — whichever is greater. State law doesn't let any town cap ADU size below 750 sq ft.
Attached or detached (detached exempt from 1-home-per-lot). Since July 1, 2025, state law allows detached ADUs by right statewide — town provisions that prohibited them are superseded.
Required on the lot — no ADU count stated; state caps at 1. 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.