Attached vs. Detached ADU in NH: Cost, Privacy, and Which Fits Your Lot

If budget is your first filter, an attached ADU is usually the cheaper build; if privacy is, a detached ADU usually wins. And since July 1, 2025, New Hampshire law lets you build either one by right — under RSA 674:72 (amended by House Bill 577, 2025 Chapter 197), every town must 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.

So the real question isn't which one you're allowed to build. It's which one your budget, your lot, and your reason for building all point to. Here's how to think it through.

What's the difference between an attached and a detached ADU?

Both are complete, independent homes — their own kitchen, bathroom, and sleeping area. The difference is where they sit:

• Attached ADU: part of the main house's structure. A finished basement apartment, an addition off the back, or a unit above an attached garage.
• Detached ADU: a standalone structure. A backyard cottage, a converted detached garage or barn, or a small new build across the yard.

New Hampshire's 2025 law treats them the same: one ADU by right, whichever form you choose. What differs is everything downstream — cost, privacy, and how your lot's constraints come into play.

Is an attached ADU cheaper to build than a detached one?

Usually, yes. An attached unit borrows what the house already has: a foundation, a roof, exterior walls, and short runs to existing water, sewer or septic, and electrical connections. A detached unit has to bring all of that with it — its own foundation and full weather envelope, plus utility lines trenched across the yard.

Typical New Hampshire ADU projects run $275,000 to $400,000+ depending on design, and the two formats tend to land at different ends of that range. On a $300k budget, an attached conversion or addition will often buy you more finished, livable space than a detached new build, because less of the money goes into sitework and structure and more goes into the living area itself.

That said, "usually cheaper" is not "always cheaper." A cramped basement that needs major egress, moisture, and ceiling-height work can cost more per usable square foot than a clean detached build on a flat, dry yard. And the garage conversion — often the most affordable path to a detached unit — gets special protection: state law allows converting an existing garage to an ADU even if the garage doesn't meet current setbacks. If you already own a sound detached garage, that can narrow the cost gap considerably.

Which one gives you more privacy?

Detached, and it isn't close. A backyard cottage has its own walls, its own entrance, and a yard between you and your tenant or family member. No shared floors carrying footsteps, no negotiating quiet hours through a wall. That separation matters most when the unit is a long-term rental, or housing an adult child who wants genuine independence.

Attached units trade privacy for proximity — which is sometimes exactly the point. If the ADU is for an aging parent, being one door away can be the whole reason to build. Worth knowing either way: state law says a town can't force the ADU to share the main home's electrical service — a separate connection or meter is the owner's choice — and can't limit the unit to one bedroom or require the occupant to be a family member. The privacy-versus-proximity call is yours to make, not your town's.

What lot constraints decide it for you?

Sometimes the lot makes the decision before you do. Three things to check:

• Setbacks. A detached ADU has to fit where the town lets accessory structures sit. In Chester, confirmed with the town, that means 40 feet from the front line, 25 feet from sides and rear, 75 feet from wetlands, and at least 15 feet of separation for a detached unit. On a small or oddly shaped lot, the buildable envelope for a standalone structure can shrink to nothing — while an attached unit rides on the house's existing footprint.
• Septic capacity. Outside sewered areas, the lot's septic system has to support a second dwelling. Added bedrooms mean added load, and that applies to both formats — but a detached unit's placement can also be constrained by where the system and its reserve area sit in the yard.
• Size caps. Towns may cap ADU size, but never below 750 sq ft. The caps vary: Amherst allows up to 1,100 sq ft, Chester up to 1,000, and Hollis caps units at 800 — and Hollis also limits an attached unit to a share of the whole dwelling's heated floor area, which on a modest house can squeeze an attached unit harder than a detached one.

None of these are reasons a town can deny the ADU itself. They're the "reasonable standards" — setbacks, lot coverage, septic, building and fire code — that still apply to any project, and they're exactly where the attached-versus-detached answer usually hides.

Are both really by right in every NH town?

Yes — since July 1, 2025, statewide. But many towns' posted ordinances haven't caught up, and the lag hits detached units hardest, because detached was the format towns historically restricted. Two examples from our Hillsborough County reviews:

• Amherst's ordinance still says a detached accessory apartment needs a discretionary Planning Board hearing. State law now guarantees that one detached ADU by right, so the hearing requirement no longer controls.
• Hollis revised its ordinance in April 2026 and still requires double the district's minimum lot size — effectively 4 acres in its main residential districts — before a detached ADU is even eligible. State law overrides that gate too.

If your town's paperwork or counter clerk says "detached isn't allowed here" or "you'll need a special exception," that's often the old ordinance talking. Our plain-English summary of the state law covers what towns can and can't require now.

How do you decide? A quick framework

• Choose attached if: budget is the binding constraint; the unit is for a parent you want close; your lot is small, tight on setbacks, or has limited septic placement options; or your house has convertible space (basement, attic, attached garage) in good condition.
• Choose detached if: privacy or rental appeal is the priority; you have the yard room and setback clearance; you already own a detached garage or barn worth converting; or your town's attached-unit rules (like a percentage-of-house cap) squeeze the size you need.
• Either way, verify before you design: your town's size cap, setbacks, septic situation, and owner-occupancy rule shape the project more than the attached-versus-detached label does. Every covered town has a breakdown in our town guides, including where the town's posted rules are superseded by the 2025 law.

The state settled whether you can build. Your lot and your budget settle which — and both answers are checkable before you spend a dollar on plans.