| Max size | 30% of main houseno fixed sq ft ceiling stated |
|---|---|
| Min size | No town minimum800 sq ft floor protects you |
| Bedrooms | Up to 2plus kitchen and bathroom |
| Type | Attached + detachedDetached: barn/garage style only |
| Parking | On-site, no count statedmust serve both units |
| 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 Mont Vernon:
Mont Vernon's ordinance requires ADU parking to be on-site, period. State law says that if the town requires extra parking for the ADU, the homeowner gets to choose whether it goes on their own lot or at a dedicated off-site spot — Mont Vernon's ordinance doesn't offer that choice.
Mont Vernon mandates an interior connecting door between every attached ADU and the main house. State law says the choice between a shared entrance and a fully separate one belongs to the homeowner, not the town.
Mont Vernon doesn't ban detached ADUs, but it confines new (non-conversion) detached ADUs to a barn/garage look — a design mandate stricter than what applies to the main house. Whether that crosses into an unlawful extra restriction under the new aesthetic-parity rule is an attorney call.
If your garage or barn already sits inside Mont Vernon's setback line because it was built decades ago, the ordinance as written doesn't clearly let you convert it into an ADU without a variance. State law says you're entitled to do that conversion regardless of the pre-existing nonconformity.
Under RSA 674:71-73 (as amended by HB 577, effective July 1, 2025), one ADU — attached or detached — is allowed by right statewide wherever single-family homes are permitted, and where a town rule is stricter, state law overrides it. Mont Vernon's Accessory Dwelling Unit ordinance (Zoning §I-308) already largely works that way: it allows one ADU per lot, attached or detached, approved administratively by the Building Inspector — no special exception or ZBA hearing required. It already gets several things right that many pre-HB577 towns get wrong: it lets you build the ADU at the same time as a brand-new principal house (not just as a retrofit), it explicitly excludes the ADU from minimum-lot-size calculations (no acreage gate), it caps bedrooms at two (not one), and its 800 sq ft size floor is actually more generous than the state's new 750 sq ft minimum. But §I-308 was last substantively amended March 14, 2023 — well before the July 1, 2025 effective date of RSA 674:71-73 as amended by HB 577 — and the town's most recent (March 2025) zoning changes only added unrelated junkyard and RV-occupancy sections, leaving the ADU rules untouched. As a result, several provisions now lag the state floor: (1) the ordinance requires ADU parking to be on-site only, with no mention of the off-site option state law guarantees; (2) attached ADUs must have an interior connecting door to the main house, when state law says the choice of a shared vs. separate entrance is the homeowner's; (3) new detached ADUs (not conversions of an existing structure) must be built to look like a barn or garage, a design mandate that may exceed what state law now allows since it isn't applied to the principal house; and (4) there's no explicit exemption letting an existing nonconforming accessory structure (e.g. a garage that already sits inside the setback) be converted into an ADU, which state law now requires. Owner-occupancy is required in one of the two units (town doesn't specify which — compliant), and the owner must record a notarized 'Notice' at the Hillsborough County Registry of Deeds barring separate condo conveyance or subdivision of the ADU. See conflicts[] for full citations; the town has not yet updated these provisions, and where they conflict with the state floor, state law controls.
Primary source: Town zoning ordinance (PDF) — §I-308 Accessory Dwelling Units last amended March 14, 2023
Get a free personalized report for your address in Mont Vernon — including where the 2025 law works in your favor.
Get My Free Mont Vernon ADU Report →House Bill 577 (2025) amended RSA 674:72, effective July 1, 2025. Wherever Mont Vernon's ordinance is stricter than this floor, the state law controls. Read the full NH ADU law explainer →
Yes — Mont Vernon allows one accessory dwelling unit (attached or detached) by right, approved administratively by the Building Inspector. 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.
30% of main house — no fixed sq ft ceiling stated. State law doesn't let any town cap ADU size below 750 sq ft.
Attached + detached (Detached: barn/garage style only). Since July 1, 2025, state law allows detached ADUs by right statewide — town provisions that prohibited them are superseded.
On-site, no count stated — must serve both units. 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.