How Much Does a Garage Conversion Cost in New Hampshire? (2026 Numbers)

A garage conversion is almost always the least expensive way to add an accessory dwelling unit in New Hampshire. A full ADU build here typically runs $275,000 to $400,000+ depending on design, and a garage conversion sits at the budget end of that spectrum — because the most expensive parts of any build, the foundation, framing, and roof, already exist.

Exactly where your project lands inside that range depends on a short list of variables: how your garage was built, how far it sits from your water, sewer, and electrical lines, and whether your lot is on town sewer or a private septic system. This guide walks through each one, plus the 2025 state law that made garage-conversion ADUs legal by right in every New Hampshire town.

Why does converting a garage cost less than building a new ADU?

When you build a detached ADU from scratch, you pay for everything: excavation, a foundation pour, framing, sheathing, a roof, and all the site work that comes with a new structure. A garage conversion skips most of that list.

• The shell is done. Slab, walls, and roof exist. You're finishing a building, not erecting one.
• Site work is minimal. No excavation, no new foundation, no new roofline to tie in — and the driveway already leads to the door.
• The permitting posture is simpler. Under state law, converting an existing garage to an ADU is allowed even if the garage doesn't meet current setbacks — a protection new construction doesn't get.

What you're really paying for in a conversion is the gap between "a box that stores cars" and "a dwelling a person can legally and comfortably live in through a New Hampshire winter." That gap is where every dollar goes.

What actually drives the cost of a garage conversion?

Two garages that look identical from the street can produce very different budgets. These are the line items that separate the low end from the high end:

• Insulation and air sealing. Garages are built as cold storage. Walls, ceiling, and — the one people forget — the slab all need insulation to meet residential energy code. An uninsulated slab in a NH winter is a deal-breaker for comfort.
• The floor itself. Garage slabs typically slope toward the door for drainage. Leveling, insulating, and finishing that floor is real work, not a cosmetic step.
• Closing the garage door opening. The big door comes out; framed wall, insulation, and usually windows or an entry door go in.
• Kitchen and bathroom. This is routinely the largest driver. An ADU needs plumbing where a garage has none, and the cost scales with distance: a garage attached to the house near existing plumbing is a short run; a detached garage across the yard means trenching water and waste lines.
• Heat and electrical. The unit needs its own heat source rated for our climate, and the existing panel may need upgrading. Note that state law leaves the electrical setup to you — a town cannot force the ADU to share the main home's service; a separate connection or meter is the owner's choice.
• Windows and egress. Any bedroom needs code-compliant egress, and most garages need added glazing for light and ventilation anyway.
• Septic. The wildcard, covered below — it can swing a budget more than every finish decision combined.

Is a garage conversion ADU even legal in New Hampshire?

Yes, in every town. Under RSA 674:72 (amended by House Bill 577, 2025 Chapter 197), effective July 1, 2025, one ADU — attached or detached — is allowed by right wherever single-family homes are permitted. No special exception, no variance, no conditional-use permit for the ADU itself. You pull a normal building permit and meet reasonable local standards: setbacks, lot coverage, septic capacity, building and fire code.

The law is unusually friendly to garage conversions specifically:

• Converting an existing garage is allowed even if the garage doesn't meet today's setbacks — old garages sitting near the lot line are protected.
• A town may cap ADU size, but never below 750 sq ft.
• At most one extra parking space can be required, and you choose on-site or off-site — convenient when your project is consuming the garage.
• The town cannot require the occupant to be a family member or limit the unit to one bedroom.
• A town may require owner-occupancy of the property, but you pick which unit you live in.

The full statewide breakdown is in our NH ADU law guide.

What's the septic wildcard?

A second dwelling has to be supported by the lot's wastewater capacity. If you're on town sewer, this is usually a straightforward connection question. If you're on a private septic system — as much of New Hampshire is — the added bedroom(s) may require a capacity evaluation, and if the existing system can't support the extra load, a septic upgrade can become the single largest line item in the entire project. Get a septic evaluation before you design anything. It's the one variable that can move a garage conversion from the budget end of the spectrum toward the middle of it.

Is a garage conversion worth the money?

Run it against the alternatives. A new-build detached ADU is a $300k-class project. A garage conversion targets the same outcome — an independent dwelling on land you already own — for the low end of the range, because you're not buying a structure, you're upgrading one.

Then look at what the unit replaces. If the plan is housing a parent, assisted living in New Hampshire commonly runs $70,000–$100,000+ per year, per commonly cited industry surveys. Against that recurring cost, a one-time conversion that keeps family on your property pays for itself quickly. If the plan is rental income, a legal, permitted ADU in a state with tight housing supply is a durable asset — and unlike a bigger house, it produces income.

How do homeowners pay for a garage conversion?

Most projects at this scale are equity-financed. The common routes:

• HELOC — a revolving line against your home's equity; you draw as construction bills come due.
• Home-equity loan — a fixed lump sum and fixed payment; simpler if your budget is nailed down.
• Cash-out refinance — replace your mortgage with a larger one and take the difference in cash.
• Construction or renovation loan — some lenders will lend against the post-project value of your property, and some count projected ADU rent when qualifying you.

Which one wins depends on your current mortgage rate and equity position. Talk to a local lender who knows ADUs — the ones who've financed them before will know how to count the future unit in your favor.

Will my town make this harder than state law says?

It shouldn't, but many posted ordinances haven't caught up to the 2025 law. Two examples from our Hillsborough County reviews: Amherst's posted ordinance still routes any detached ADU — converting a detached garage included — through a discretionary Planning Board hearing, and Hollis's still says a detached ADU, converted garage or new build, needs a lot twice the district minimum, four acres in some districts. State law overrides both, but a homeowner reading the town PDF would never know it. Before you budget a dollar, check your town's page in our town guides to see which local rules stand and which are superseded.

The cheapest ADU you'll ever build is the one whose walls are already standing — go find out what your garage is worth.