How Big Can a Detached ADU Be in NH? Size Caps, Town by Town

In every New Hampshire town, you're entitled to build a detached ADU as large as 750 square feet — no local size cap can be set below that number since RSA 674:72 (amended by House Bill 577, 2025 Chapter 197) took effect July 1, 2025. Above that floor, the ceiling is the town's call: some towns stop at exactly 750 sq ft, a large cluster lands at 950, and the most generous allow 1,000, 1,100, 1,400, even 1,500 square feet.

That spread is the single biggest practical difference between building an ADU in one NH town versus the next. Here's how the numbers break down across the towns in our guides.

What does the 750 sq ft "floor" actually mean?

It's the most misread number in the whole law, so let's be precise. Under RSA 674:72, a town may cap the size of an ADU — but that cap can never be set below 750 square feet. In other words, 750 is the smallest maximum a town is allowed to impose. It is not a minimum: state law does not require your ADU to be any particular size, and a town that stays silent on minimums (most do) leaves you free to build a compact unit.

Two things follow from that:

• If your town's ordinance caps ADUs below 750 sq ft, the cap is superseded — you're entitled to 750 anyway.
• If your town's ordinance allows more than 750, that generosity stands. State law sets a floor under the cap, not a ceiling over it.

One practical caveat: the floor governs the town's size cap, not your lot — ordinary standards like setbacks, lot coverage, and septic capacity still shape what actually fits.

Some towns do set their own minimum sizes. That's a town choice, not state law — more on those below.

Which towns cap detached ADUs at exactly 750 sq ft?

A handful of towns sit right at the least generous cap the law allows:

• Hooksett adopted a flat 750 sq ft cap on habitable area at its March 10, 2026 vote — the same ballot article that legalized detached ADUs there. Notably, the new flat cap is actually tighter for owners of larger homes than the percentage formula it replaced.
• Weare caps accessory living space at 750 sq ft. Its posted ordinance still says an ADU must be built within or attached to the house — language state law now overrides for your one by-right detached unit.
• Strafford caps ADUs at 750 sq ft with no attached-versus-detached distinction.
• Hudson caps ADUs at 750 sq ft (with its own 350 sq ft minimum). Its posted ordinance still expressly prohibits detached ADUs — a pre-2025 provision the state law supersedes.

In these towns, the town's number and the state's floor are the same thing: plan around 750 square feet.

Is 950 sq ft a common cap?

In our town-by-town reviews, 950 comes up again and again — several towns rewrote their ordinances after the 2025 law and landed on that figure:

• Windham — 950 sq ft of occupied floor space, adopted March 10, 2026, alongside newly legalized detached units.
• Boscawen — 950 sq ft of gross living area under its 2026 amendment, which also deleted the old 75%-of-the-primary formula so the cap can never compute below the state floor.
• Auburn — 950 sq ft, confirmed with the town, in an ordinance that tracks the state law's structure closely.
• Concord — a 950 sq ft base cap that can stretch up to 1,250 sq ft, so long as the ADU is no more than half the main home's floor area. Concord's old 750-max/600-minimum rule is gone; if you see those numbers in a cached copy of the code, you're reading a superseded version.

Which towns allow the biggest detached ADUs?

This is where town choice really shows. Several towns authorize far more than the typical cap:

• Antrim — up to 1,500 sq ft, confirmed with the town, so long as the ADU doesn't exceed 50% of the primary home's gross living area. Antrim also allows two by-right ADUs — one attached, one detached — which makes it one of the most ADU-friendly towns we've reviewed.
• Alton — up to 1,500 sq ft of gross floor area, limited by percentage-of-the-house formulas, with the ordinance's own promise that the town will never require an ADU to come in under 750 sq ft.
• Hopkinton — the lesser of 1,400 sq ft or 50% of the principal dwelling's floor area, and if that 50% math falls below 750, the cap bumps back up to 750.
• Amherst — up to 70% of the primary home's heated floor area, maxing out at 1,100 sq ft, confirmed with the town.
• Brentwood — the lesser of 1,100 sq ft or 50% of the primary home's size, confirmed with the town. On smaller homes that formula can compute below 750 — and where it does, the state floor controls.
• Atkinson — a straight 1,000 sq ft cap, confirmed with the town.
• Stratham — 1,000 sq ft, though a detached unit is also capped at half the primary home's interior space, and the figure includes appurtenant space like garages, porches, and decks, which effectively shrinks the living area.
• Mont Vernon — no fixed ceiling at all: the cap is 30% of the primary home's gross living area, so a large house can support an ADU above the caps most towns use.

The pattern worth noticing: several generous towns pair a big headline number with a percentage-of-the-house formula. Your real cap depends on your primary home's size, so run both numbers before you sketch a floor plan.

Do some towns set a minimum ADU size?

Yes — and this is purely a town invention, since state law sets no minimum anywhere. From the towns we've reviewed:

• Brentwood requires at least 720 sq ft.
• Stratham requires 400 sq ft (its ordinance states a 400–1,000 range).
• Hudson requires 350 sq ft.
• Dover requires 300 sq ft (within its 800 sq ft cap).
• Mont Vernon guarantees you can build at least 800 sq ft even when its percentage formula computes lower — a town-set floor more generous than the state's, not a required minimum.

If you're planning a deliberately small unit, check your town's page first — a minimum can matter as much as a cap.

What if my town's cap is below 750?

Then the state wins. Windsor is the clearest example among the towns we've reviewed: its posted ordinance caps ADUs at 500 square feet, but that number is below the legal floor and no longer enforceable — a Windsor homeowner is entitled to at least 750. The same logic retires the sub-750 caps still sitting in older, superseded ordinance text elsewhere. This is exactly why reading the town's posted PDF at face value can cost you square footage you're legally entitled to — our town guides read each ordinance side-by-side with the state law so you don't have to.

How much does the size cap change the budget?

Meaningfully. A typical NH detached ADU runs $275,000–$400,000+ depending on design, and square footage is one of the biggest levers in that range. A town that allows 1,400 sq ft gives you room for a true two-bedroom layout; a 750 sq ft town pushes you toward an efficient one-bedroom plan. Before you fall in love with a design, confirm your town's exact cap — start with your county guide and open your town's page.

The state guarantees the floor; your town sets the ceiling — and now you know exactly where yours is.