| Max size | 1,500 sq ftor 35% of main house, whichever less |
|---|---|
| Min size | No town minimum750 sq ft floor protects you |
| Bedrooms | Up to 2 |
| Type | Attached or detachedLakeshore: special exception |
| Parking | 4 off-street spaceshouse + ADU combined, on-site |
| 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 Alton:
Since July 1, 2025, state law entitles you to a by-right building permit for one ADU in any district that allows single-family homes. Alton's ordinance still forces a hearing in front of the Zoning Board for an ADU in the Lakeshore Residential District — but that requirement is no longer legal; you're entitled to a by-right permit, not a discretionary board vote.
State law now caps what the town can require at one additional parking space for the ADU, and you — not the town — choose whether that space is on-site or off-site. Alton's ordinance still bundles a flat 4-space requirement for the house-plus-ADU combined; unless Alton's general single-family parking requirement is already 3 spaces (unlikely), that exceeds the one-extra-space cap and is overridden by state law.
Under the 2025 state law, the choice of where an ADU's entrance goes belongs to you, the homeowner. The town's rule that any new ADU entrance must go on the side or back of the house — never the front — is exactly the kind of ingress/egress dictate the law took off the table.
Alton's clause doesn't (unlike some towns) explicitly call the interior door the ADU's PRIMARY means of access — it just requires that a connecting door exist. Whether mandating any interior door at all crosses the line into limiting entrance choice is a judgment call an attorney should make.
The town can only hold your ADU to the same look-and-feel standard as a regular house — not a stricter one. Whether "maintain aesthetic continuity" crosses that line depends on how it's enforced in practice; an attorney call.
State law guarantees you one ADU — attached or detached, new construction or conversion — by right. It isn't clear from the ordinance's text whether Alton would issue a permit for a brand-new, purpose-built detached ADU (not a conversion of something already standing) — the wording only clearly covers converting an existing accessory structure. If Alton's practice is conversion-only, that falls short of the state's detached-by-right mandate and should be confirmed directly with the town.
If your existing garage sits closer to the property line than §327's setback rules allow, state law says the town cannot block converting it into an ADU on that basis — even though Alton's ordinance, as written, doesn't spell out that exemption itself.
Start with what state law guarantees you: since July 1, 2025, New Hampshire law (RSA 674:71-73, as amended by HB 577) entitles every homeowner to one accessory dwelling unit — attached or detached — by right, wherever single-family homes are permitted. No special exception or conditional-use hearing may be required for that one by-right ADU, the town may require at most one additional parking space (on-site or off-site, your choice), and the town cannot dictate your entrance location. Alton's ADU section (Zoning Ordinance §319) was last amended 13 March 2018 — roughly seven years before that state law took effect — and has not been reconciled to it, so several of its provisions are now overridden. What the town's ordinance says: one ADU per lot, attached to the house or within an accessory structure, up to 1,500 sq ft (or 35% of the principal dwelling's floor space, or 50% of an accessory structure's floor space, whichever is smaller) but never required to be under 750 sq ft; likely a 2-bedroom cap; owner-occupancy required in either unit; a sewage-disposal approval requirement tied to state law; and a flat ban on condo-converting the property or attaching ADUs to townhouse-style homes or manufactured housing. Where the ordinance's stale text is now overridden by the state-law floor: (1) the ordinance still requires a discretionary Special Exception hearing before the Zoning Board for an ADU in the Lakeshore Residential District — state law entitles you to a by-right permit in any district that allows single-family homes; (2) the ordinance still requires FOUR total off-street parking spaces for the house-plus-ADU combined with no on-site/off-site choice — state law caps the ADU-specific add at one extra space, chosen by the homeowner; (3) the ordinance still says any new entrance/exit added for the ADU must go on the side or rear of the house — state law bars towns from dictating entrance location. Two more provisions are genuinely ambiguous and worth a second look: a mandatory interior connecting door between attached units, and a requirement that the ADU maintain 'aesthetic continuity' with the main house — both echo language the state law now limits, but neither is unambiguous enough to call without an attorney. Finally, the ordinance's phrasing around detached ADUs reads more like a conversion-of-an-existing-structure allowance than a green light for new detached construction, which may fall short of the state's detached-by-right mandate — confirm this directly with the Alton Land Use office before planning a new-build detached unit.
Get a free personalized report for your address in Alton — including where the 2025 law works in your favor.
Get My Free Alton ADU Report →House Bill 577 (2025) amended RSA 674:72, effective July 1, 2025. Wherever Alton's ordinance is stricter than this floor, the state law controls. Read the full NH ADU law explainer →
Yes — and under state law effective July 1, 2025, you're entitled to one ADU by right (attached or detached) anywhere in Alton that single-family homes are allowed. Alton's own ordinance still layers a Special Exception hearing onto the Lakeshore Residential District and several other outdated rules, but state law now overrides those. 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.
1,500 sq ft — or 35% of main house, whichever less. State law doesn't let any town cap ADU size below 750 sq ft.
Attached or detached (Lakeshore: special exception). Since July 1, 2025, state law allows detached ADUs by right statewide — town provisions that prohibited them are superseded.
4 off-street spaces — house + ADU combined, on-site. 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.