Attached vs. Detached Garage Conversion in NH: Which Is the Easier ADU?

For most New Hampshire homeowners, converting an attached garage is the easier path to an ADU: the water, sewer, and electrical service are already a few feet away, one or two walls are shared with heated space, and the whole project stays under your existing roofline. A detached garage conversion is entirely doable — and since July 1, 2025, both are legal by right statewide under RSA 674:72 (amended by House Bill 577, 2025 Chapter 197) — but it behaves more like a small standalone build, with the trenching, full building envelope, and independent systems that come with it.

Here's how the two compare on the things that actually drive difficulty and cost.

Are both garage conversions really allowed by right?

Yes. State law requires every NH town to allow one ADU — attached or detached — by right wherever single-family homes are permitted. No special exception, no variance, no conditional-use permit for the ADU itself. You still need a normal building permit, and reasonable local standards (setbacks, lot coverage, septic capacity, building and fire code) still apply.

The law also includes a protection written almost specifically for garage conversions: converting an existing garage to an ADU is allowed even if the garage doesn't meet current setbacks. That matters because older garages — especially detached ones — often sit closer to the lot line than today's rules would allow. Under the state law, that nonconformity can't be used to block the conversion. The full statewide picture is on our NH ADU law page.

How is the converted unit classified — attached or detached ADU?

Simply: the unit takes the classification of the structure it lives in.

• Convert a garage that's physically connected to your house (shared wall, breezeway, or under the same roof) and the result is generally treated as an attached ADU.
• Convert a freestanding garage and you've created a detached ADU.

Before July 2025, that classification mattered enormously — towns had to allow attached ADUs but could prohibit or gate detached ones. Now the state floor is the same for both: one ADU by right either way. Some town ordinances make the equivalence explicit. Manchester's ordinance, for example, lists an ADU "within or above its attached or detached garage" right alongside in-home and standalone units as permitted locations.

Where classification still matters is in the details a town can lawfully vary: detached units are typically measured against accessory-structure setbacks, and a few towns write different dimensional standards for detached ADUs. Your town guide will show which bucket your project falls into.

Which conversion is cheaper to run utilities to?

This is usually the single biggest cost gap between the two.

• Attached garage: water, drain lines, and the electrical panel are typically on the other side of a wall. Tying in is measured in feet of pipe and wire, not trench.
• Detached garage: water and sewer (or a septic connection) have to be trenched from the house or the street, buried below frost depth, and the electrical feed run underground or overhead to a subpanel. Every foot of distance between house and garage adds cost.

One thing the state law settles in your favor either way: a town cannot force the ADU to share the main home's electrical service. A separate connection or meter is the owner's choice — useful if you plan to rent the unit and want clean utility billing.

Septic is the other gate. A second dwelling has to be supported by the lot's septic capacity or sewer connection regardless of which garage you convert, so in unsewered towns the septic evaluation should be one of your first calls.

What about fire separation between the units?

Here the attached conversion picks up work the detached one avoids. When two dwelling units share a wall, floor, or ceiling, building and fire codes require rated separation between them — and the existing wall between a garage and a house was built to garage-separation standards, not dwelling-unit-separation standards, so expect it to be upgraded.

A detached conversion has no shared assembly with the main house, so unit-to-unit separation largely drops away. The trade-off: if the freestanding garage sits close to a property line, code can require rated exterior walls or limit openings on that side. Your building department will tell you exactly what applies — these are code questions, not zoning ones, and they're project-specific.

How much insulation work does a garage conversion need?

Both conversions need real envelope work, because garages are built as cold storage: slab floors with no insulation beneath, thin or empty walls, and often no ceiling insulation at all. Plan on insulating the slab or building an insulated floor over it, bringing walls and roof up to habitable standards, and swapping the garage door opening for an insulated framed wall with proper windows.

The attached conversion has a head start — one or two walls already border heated space, and you may be able to extend the house's existing heating system. The detached conversion needs the full six-sided envelope plus its own heat source, which in practice often means a cold-climate heat pump. In New Hampshire's climate, skimping here is the classic garage-conversion mistake.

What are the egress requirements?

Every sleeping room needs a code-compliant emergency escape opening, and the unit needs proper exit doors — which usually means new or enlarged windows, since garage windows (when they exist) are rarely sized for egress. Plan for the bedroom count you actually want: state law says a town can't limit the ADU to one bedroom. Confirm the specific window and door requirements with your building inspector — they're set by building code, not the ADU statute.

So which one actually costs less?

Qualitatively: the attached conversion, in most cases. Shorter utility runs, shared walls, a possible heating tie-in, and no new site work add up. The detached conversion buys you something the attached one can't, though — real separation. Privacy for a renter, independence for an aging parent, a true backyard cottage feel.

Either conversion typically lands well below a ground-up ADU, which in New Hampshire commonly runs $275,000–$400,000+ depending on design. How far below depends on the garage's condition, the trench distance, and how much structure you're reusing — get local bids before anchoring on a number.

Does the answer change town to town?

The by-right floor doesn't, but the details do — and some posted ordinances still lag the state law:

• Auburn rewrote its ordinance to match the law cleanly: ADU conversions of existing structures, "including but not limited to detached garages," are expressly allowed even where the structure violates current setback or lot-coverage requirements.
• Amherst's posted ordinance still routes detached ADUs — a converted freestanding garage included — through a discretionary Planning Board hearing, a requirement the state law now overrides.
• Manchester allows garage ADUs explicitly, but its setback exemption as written applies to structures built before July 1, 2025 — a cutoff the statute itself doesn't contain, and worth confirming for a newer garage.
• Hollis's ordinance still ties detached-ADU eligibility to doubled lot size, another condition the state law supersedes.

Every covered town has a plain-English breakdown on this site showing which local rules stand and which are superseded.

Whichever garage you own is the one worth converting — the law now says yes to both, and the rest is a construction question, not a permission question.