Detached ADUs Are Now Legal By Right in Every New Hampshire Town

Yes — you can build a detached accessory dwelling unit on your single-family lot in New Hampshire, in every town, as a matter of right. Since July 1, 2025, state law (RSA 674:72, amended by House Bill 577) requires every municipality to allow one ADU — attached or detached — wherever single-family homes are permitted, with no special exception, variance, or conditional-use permit for the ADU itself.

That single sentence is the biggest change to New Hampshire residential zoning in a generation. Before it, a backyard cottage was legal in some towns, special-permission-only in many, and flatly prohibited in others. Now the question isn't whether you can build one — it's what the details look like in your town.

What "by right" actually means

"By right" (your town may say "permitted use") means the use itself cannot be denied. You still apply for a normal building permit, and your project still has to meet the town's reasonable standards — setbacks, lot coverage, septic capacity, building and fire codes. What the town can no longer do is make the existence of your detached ADU discretionary: no zoning board hearing to plead your case, no conditional-use findings, no neighbor-approval gauntlet.

Before and after: what HB 577 changed for detached units

• Before July 1, 2025: Towns had to allow attached ADUs, but detached units were optional — each town chose whether to allow them at all, and many that did required a special exception or conditional-use permit.
• After July 1, 2025: One ADU, attached or detached, is allowed by right on single-family lots statewide. A town may still apply its normal dimensional standards, but it cannot prohibit detached units or route them through discretionary approval.
• Also protected: converting an existing garage to an ADU is allowed even if the garage doesn't meet today's setbacks; towns can't require the occupant to be family; can't limit the unit to one bedroom; can't force you to share the main home's electrical service; and can cap size — but never below 750 sq ft.

Why your town's website may still tell you no

Zoning ordinances get amended at town meetings, once a year in most towns. Many municipalities haven't caught their posted ordinance up to the state law — and some towns that have amended their ADU sections since 2025 still kept language the state law overrides. Real examples from our town-by-town review:

• One Rockingham County town reaffirmed a "2026 revision" of its ordinance that still mandates a shared street entrance and on-site-only parking — both stricter than what state law now allows.
• A Hillsborough County town revised its ADU ordinance in April 2026 — well after the new law — and still kept conflicting clauses on the books.
• A seacoast village district's posted ordinance still prohibits detached ADUs outright and allows ADUs only by special exception; its rewrite hasn't yet been adopted, so the conflict is live today.

In every one of those cases, the state law controls. But a homeowner reading the town's posted PDF — or talking to a counter clerk who's quoting it — would walk away believing a detached ADU is off the table. That's exactly the gap this site exists to close: every town guide on NH ADU Guide reads the town's current ordinance side-by-side with RSA 674:72 and tells you which rules stand and which are superseded.

What towns can still regulate on a detached ADU

The floor is statewide; the details are local. Legitimate town-by-town differences include:

• Size cap — towns may cap ADU size, but no lower than 750 sq ft; some allow much more.
• Setbacks and lot coverage — your detached unit generally has to sit where an accessory structure may sit (with the garage-conversion exception noted above).
• Parking — at most one additional space, and you choose whether it's on-site or off-site.
• Owner-occupancy — a town may require the owner to live on the property, but you pick which unit you live in.
• Septic and utilities — a second dwelling has to be supported by the lot's septic capacity or sewer connection.

How to find your town's exact rules

Every covered town has a plain-English breakdown on this site — size caps, parking, owner-occupancy, and every place the town's posted rules are superseded by the 2025 law. Start with your county guide, open your town, and if you want the specifics for your own property, request the free personalized report. If your town's numbers carry the "verified by a real human" badge, they've been confirmed directly with the town office.

The law finally says yes. The details are what you build with — and they're one click away.