Backyard Cottages in New Hampshire: Rules, Sizes, and Setbacks Explained

A backyard cottage in New Hampshire is, legally speaking, a detached accessory dwelling unit — a detached ADU — and since July 1, 2025 you can build one by right on any lot where a single-family home is permitted, in every town in the state. That guarantee comes from RSA 674:72, amended by House Bill 577 (2025 Chapter 197), and it applies whether your town calls the unit an ADU, an accessory apartment, or nothing at all.

The words matter, because "backyard cottage" appears almost nowhere in New Hampshire zoning ordinances. Once you know what your town calls it, the rules snap into focus.

What is a backyard cottage called in a zoning ordinance?

Homeowners say backyard cottage, granny flat, in-law cottage, guest house, carriage house, casita. Zoning ordinances say accessory dwelling unit — a second, smaller dwelling on the same lot as a single-family home. When it's a standalone building rather than an addition or a basement apartment, it's a detached ADU. That's your backyard cottage.

Towns add their own flavor. Hollis labels ADUs "in-law apartments" in its ordinance. Meredith calls the same thing an "Accessory Apartment." Different names, same legal creature — and the same state law sits above all of them.

One practical note: what makes a backyard building a dwelling rather than a shed or studio is generally that it's set up for someone to live in — sleeping space, kitchen, bathroom. If your cottage will have those, expect your town to treat it as an ADU and apply the ADU rules. That's usually good news, because the ADU rules now come with state-law protections a plain accessory building doesn't get.

Are backyard cottages legal in New Hampshire?

Yes — everywhere single-family homes are, and by right. Under RSA 674:72 as amended, effective July 1, 2025, every municipality must allow one ADU, attached or detached, wherever single-family homes are permitted. No special exception, no variance, no conditional-use permit for the ADU itself. You apply for a normal building permit, and your project meets the town's reasonable standards — setbacks, lot coverage, septic capacity, building and fire codes — the same categories of rules any house has to meet.

Before July 2025, that wasn't true. Towns had to allow attached ADUs, but a detached cottage was each town's choice, and many either banned them or routed them through a zoning board hearing. The state law page covers the full before-and-after.

How big can a backyard cottage be?

State law sets one hard rule: a town may cap ADU size, but never below 750 square feet. There is no state minimum size — though some towns impose their own minimum, and that's a town choice, not state law.

The real answer is town by town, and the spread is wide. From our town guides:

• Hollis allows ADUs between 300 and 800 square feet.
• North Hampton caps a cottage at an 800-square-foot footprint — measured to the outside of the walls, with every floor counted against the same total — and also requires at least 720 square feet of ground-floor living area. That squeezes every legal ADU there into a narrow band and effectively rules out a small studio cottage.
• Meredith's ordinance allows up to 1,200 square feet, subject to a percentage formula tied to the main house.

Two towns, twenty minutes apart, can point you toward completely different buildings. Check your town's guide before you fall in love with a floor plan.

Where on the lot can a backyard cottage go?

Placement is governed by setbacks — the minimum distances a structure must keep from your property lines — plus lot coverage limits and, on well-and-septic lots, where your septic system can support a second dwelling. These are local numbers, and they're legitimate: state law lets towns apply reasonable dimensional standards to ADUs just as they do to houses.

What placement looks like in practice:

• Setbacks vary by town and district. In Hollis's main residential district, for example, the dimensional table sets a 50-foot front setback, with side and rear setbacks of 35 feet for a principal structure and 15 feet for accessory structures — and which of those numbers applies to your cottage is a question to settle with the building department before you draw plans. Your town's numbers may be nothing like that — pull them from your town guide or the dimensional table in your ordinance.
• Height limits are usually the ordinary district limit. North Hampton applies its standard 35-foot cap; Hollis's districts run to 38 feet. Most cottages come nowhere near either.
• Garage conversions get special protection. State law says converting an existing garage to an ADU is allowed even if the garage doesn't meet current setbacks. If your old detached garage sits close to the line, that's not a dealbreaker — it's the one placement rule the state settled in your favor statewide.
• Watch for leftover siting rules. Meredith's ordinance still says a detached unit must share its building with another accessory use, like a garage or workshop, and sit within 150 feet of the main house. The state's detached-by-right guarantee doesn't carry strings like that shared-building requirement, so state law overrides it — but whether a distance limit like the 150-foot rule survives the 2025 law is a genuinely open question, one to walk through with the town (and, on a close call, an attorney) before you site the building.

What other rules apply to a backyard cottage?

Beyond size and placement, the short list looks like this:

• Parking: a town may require at most one additional space, and only if it already requires parking for the main house — and you choose whether that space is on-site or off-site. Some towns skip it entirely; North Hampton requires no extra ADU parking at all.
• Owner-occupancy: a town may require the owner to live on the property, but you pick which unit you live in — the house or the cottage.
• Who can live there: towns cannot require the occupant to be a family member. "Granny flat" is a nickname, not a rule — you can rent to anyone.
• Bedrooms: towns cannot limit the cottage to one bedroom.
• Utilities: you can't be forced to share the main home's electrical service — a separate connection or meter is your choice. Septic or sewer capacity for the second unit still has to check out.

Why does my town's ordinance say backyard cottages aren't allowed?

Because many posted ordinances haven't caught up. Zoning gets amended at town meeting, usually once a year, and plenty of towns still publish ADU rules written before July 2025 — some towns that amended afterward even kept conflicting language on the books. Meredith's ordinance still routes every accessory apartment through a zoning board special exception; Hollis still says a detached ADU needs a lot twice the district minimum — four acres in its main residential districts. Under the 2025 state law, requirements like those no longer control, but a homeowner reading the town PDF cold would never know it.

That gap is exactly what our town guides exist to close: each one reads the town's current ordinance next to RSA 674:72 and tells you which rules stand, which are superseded, and which are genuinely unsettled.

Call it a cottage, a granny flat, or a carriage house — the law calls it a detached ADU, and the law is on your side.