| Max size | 50% of main houseno absolute sq ft ceiling stated |
|---|---|
| Min size | No town minimumbuild as small as code allows |
| Bedrooms | Not statedordinance is silent on bedrooms |
| Type | Attached + detachedTown ordinance still requires a CUP |
| Parking | Off-street, no count statedmust serve both units combined |
| 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 Madbury:
This is the big one. Madbury still routes every ADU through a Conditional Use Permit hearing at the Planning Board. Since July 1, 2025, state law says one ADU — attached or detached — is yours AS A MATTER OF RIGHT. A regular house in Madbury doesn't need a CUP, so the town can't demand one for your ADU either. In practice you should still expect the town to ask for the CUP until it amends its ordinance; state law is on your side, but you may have to say so out loud.
Madbury caps your ADU at half the size of your house. If your house is under 1,500 square feet, that math pushes your ADU below 750 square feet — and state law says a town may NOT restrict an ADU to less than 750 square feet. So a 1,200 sq ft house would get a 600 sq ft ADU under Madbury's rule, but you're entitled to at least 750. Owners of larger homes are unaffected (and Madbury's percentage rule actually lets them go ABOVE 950 sq ft, which state law permits a town to authorize).
Madbury's rule is open-ended — 'the combined needs' of both units, off-street — with no number attached, and the Planning Board reviews where you put it. State law caps the ADU at ONE extra space, and the location choice (your lot, or a legally dedicated off-site spot) is YOURS, not the board's.
If you have an old detached garage or barn sitting closer to the property line than today's 15-foot setback allows, Madbury's blanket 'must meet all pertinent zoning requirements' would block you from converting it. State law flatly overrides that — garage conversions must be allowed even when the existing structure is setback- or lot-coverage-nonconforming.
Building a plain single-family house in Madbury triggers no design review and no neighborhood-character test. Running your ADU through a CUP that asks for a photo of your house and judges whether the design is 'compatible with the established character of the neighborhood' applies design review that a regular house never faces — which is exactly what the 2025 law bars. The §4.C.3 'should be developed consistent with' language is softer (hortatory, not mandatory), but the CUP criterion it feeds into is not.
Requiring septic capacity to be adequate is legitimate and state law says the same thing. The gray area is whether Madbury can demand a licensed septic designer's certification of an EXISTING, already-approved, functioning system — state law says the town can't impose septic requirements beyond what NHDES itself requires. This is a narrow question for a NH land-use attorney; it may well be compliant.
Nothing in Madbury's text explicitly bans building the house and the ADU together — but the ordinance is written entirely around adding an ADU to an existing home, and the CUP application literally asks for a photograph of the existing house. If you're building new construction with an ADU from day one, state law says you don't have to build the house first and retrofit later.
Madbury's ADU rules live in Article V, Section 4 of the zoning ordinance ('Accessory Dwelling Units'), which the town replaced wholesale at its March 11, 2025 Town Meeting ('Change 5'). Madbury deserves real credit for one thing: that 2025 rewrite explicitly allowed DETACHED ADUs ('Accessory Dwelling Units may be attached or detached') about four months before New Hampshire's HB 577 made detached-by-right the statewide rule on July 1, 2025. Several other provisions also line up cleanly with the new law — only one ADU per lot, no bedroom cap, owner-occupancy required but the owner picks which unit to live in, and (helpfully) the ADU doesn't count as an extra dwelling unit against Madbury's 80,000 sq ft minimum lot size.
But the timing is the whole story. Madbury's ADU text was frozen in March 2025 — BEFORE the state law changed — and the town did not revisit it afterward. Its one zoning amendment on the March 10, 2026 ballot (Article 02) was about something else entirely: allowing multi-family housing in the Commercial & Light Industry zone to comply with a different bill, HB 631. So as of today the posted, currently-in-force ADU ordinance has never been reconciled with HB 577, and four provisions in it are overridden by state law:
1) THE PERMIT. Madbury §4.C.8 says an ADU is 'permitted subject to the approval of a Conditional Use Permit' from the Planning Board, with an application that wants a written compliance narrative, a photo of your house's exterior, and a site sketch. State law (RSA 674:72, I) says one ADU 'shall be allowed as a matter of right.' A plain single-family house in Madbury needs no CUP, so the town cannot demand one for your ADU. This is the single biggest gap.
2) THE SIZE. Madbury §4.C.7 caps the ADU at 50% of your principal dwelling's finished living area, with no absolute square-foot number attached. If your house is under 1,500 sq ft, that formula pushes your ADU below 750 sq ft — and RSA 674:72, VII says a town 'may not restrict the total living space to less than 750 square feet.' A 1,200 sq ft house gets a 600 sq ft ADU under the town's math; state law entitles you to at least 750. (If your house is large, the town's percentage rule is actually generous — it lets you exceed the 950 sq ft state default, which towns are allowed to authorize.)
3) THE PARKING. Madbury §4.C.9 requires open-ended off-street parking 'to serve the combined needs' of both units, with the Planning Board reviewing where it goes. State law caps the extra ADU parking at ONE space and puts the on-site-vs-off-site choice in the owner's hands, not the board's.
4) THE GARAGE CONVERSION. Madbury §4.C.6 says the ADU 'must meet all pertinent zoning and building requirements,' with no exception for structures that already exist. RSA 674:72, XI requires towns to allow ADU conversions of existing structures — detached garages specifically — even when those structures violate today's setback or lot-coverage rules. Madbury's ordinance as written would block that conversion; state law does not let it.
Two softer flags: the CUP's design-review criteria (Art. IV §9 asks whether 'the proposed layout and design shall be compatible with the established character of the neighborhood,' and §4.D wants a photo of your house) apply aesthetic and design review that an ordinary house never faces — which RSA 674:72, I bars. And the ordinance is written entirely around bolting an ADU onto an existing home, while state law lets you build the ADU alongside a brand-new house in a single project.
Bottom line for a Madbury homeowner: you are entitled to one ADU — attached OR detached, up to at least 750 sq ft, with at most one extra parking space of your choosing — without a Conditional Use Permit hearing. The town's posted ordinance will tell you otherwise. Expect Madbury to still ask for the CUP until it amends the section; you should bring RSA 674:71-73 with you, and talk to a NH land-use attorney before you rely on this.
Primary source: Town zoning ordinance (PDF) — Article V, Section 4 (Accessory Dwelling Units)
Get a free personalized report for your address in Madbury — including where the 2025 law works in your favor.
Get My Free Madbury ADU Report →House Bill 577 (2025) amended RSA 674:72, effective July 1, 2025. Wherever Madbury's ordinance is stricter than this floor, the state law controls. Read the full NH ADU law explainer →
Yes — Madbury allows one accessory dwelling unit, attached or detached, in its General Residential and Agricultural District (the town's only district that permits houses). 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.
50% of main house — no absolute sq ft ceiling stated. State law doesn't let any town cap ADU size below 750 sq ft.
Attached + detached (Town ordinance still requires a CUP). Since July 1, 2025, state law allows detached ADUs by right statewide — town provisions that prohibited them are superseded.
Off-street, no count stated — must serve both units combined. 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.