Yes — you can convert your garage into an in-law apartment in New Hampshire, and in every town it now takes a normal building permit rather than a zoning board hearing. Since July 1, 2025, state law (RSA 674:72, amended by House Bill 577, 2025 Chapter 197) allows one accessory dwelling unit by right wherever single-family homes are permitted — and it specifically protects garage conversions, even when the garage doesn't meet today's setback rules.
That last part matters more than most homeowners realize. Garages are often the closest structure on the lot to a property line, and before 2025 that alone could sink a conversion. Now it can't. Here's the full walkthrough: assess, design, permit, build.
Yes. "In-law apartment," "in-law suite," "granny flat," "accessory apartment," and "garage apartment" are all everyday names for what New Hampshire law calls an accessory dwelling unit — an independent living space with its own kitchen, bathroom, and sleeping area on a single-family lot. Some town ordinances use the older vocabulary; the law treats them the same.
One important update to the "in-law" framing: the family-only rules are gone. Towns can no longer require that the person living in the unit be a relative. If your parents move out someday, you can rent the apartment to anyone. That flexibility is a big part of why a conversion pencils out — and with NH assisted living commonly running $70,000–$100,000+ per year according to commonly cited industry surveys, keeping a parent close in a private apartment is often the motivation that starts the project.
No special zoning approval — that's the change. Under the state ADU law, one ADU (attached or detached) is a by-right use on any lot where a single-family home is permitted. The town cannot require a special exception, variance, or conditional-use permit for the ADU itself. You still need a building permit, and your project still has to meet reasonable local standards: setbacks for new construction, lot coverage, septic capacity, building and fire code.
Two protections are especially relevant to garages:
Be aware that many towns' posted ordinances still contain language the state law now overrides. Amherst is a clear example: its accessory-apartment ordinance still says a detached unit — which is what a freestanding garage conversion is — needs a discretionary Planning Board hearing. State law overrides that requirement, but a homeowner reading the town PDF cold would never know it. Our town guides read each ordinance side-by-side with the state law so you know which rules stand.
Before you sketch a floor plan, get honest answers on three fronts.
Structure. A builder or structural engineer should look at the foundation and slab (cracking, frost heave, whether the slab can support interior walls and plumbing), the framing (garage walls are often framed lighter than living-space walls), the roof, and headroom after you add an insulated floor and finished ceiling. Detached garages built as cheap outbuildings sometimes need enough structural work that a homeowner is better off rebuilding on the same footprint — a good builder will tell you which side of that line you're on.
Utilities. Map what already reaches the garage: electrical (usually a small subpanel at best — a full apartment needs real capacity), water and sewer lines (often nothing, and trenching from the house is a real line item), and heat (none, typically — mini-splits are the common answer).
Septic. If you're not on town sewer, this is the first phone call, not the last. A second dwelling unit adds bedrooms to the load your septic system is designed for. Have a septic designer evaluate the existing system and, if needed, design an upgrade approved by the state. Towns are allowed to hold ADUs to septic standards, and this is the single most common reason rural conversions stall.
With the assessment done, design is where the apartment takes shape: insulation and moisture control (garage slabs and walls were never built to living-space standards), converting the vehicle door opening into a wall with windows or a slider, ceiling height and egress windows for any bedroom, and code items like fire separation from an attached house.
Design against your town's numbers from day one. Towns may cap ADU size — never below 750 sq ft, but the cap varies. In Chester, confirmed with the town, an ADU can be up to 1,000 sq ft with a 600 sq ft minimum, two bedrooms max, owner-occupancy required, and one required parking space. Amherst, also confirmed with the town, caps units at 1,100 sq ft with two bedrooms. A typical two-car garage footprint sits comfortably inside those caps, which is exactly why garages convert so well. Check your town's page in our Hillsborough County or Rockingham County guides before your designer draws a line.
Also note: towns can't limit the unit to one bedroom, and they can require at most one additional parking space — you choose whether it's on-site or off-site. Losing garage parking is usually a non-issue.
For most garage conversions, the stack looks like this:
What you should not be asked for is a zoning board hearing for the ADU itself. If a counter clerk quotes an ordinance requiring a special exception or conditional-use permit for your one by-right unit, the ordinance is out of date — the state law controls. A town may still require owner-occupancy of the property, but you pick which unit you live in.
Plan in phases rather than around a magic number. Assessment and septic evaluation typically take weeks; design and permitting a few months depending on your town's workload; and the build itself is usually the shortest phase, since the shell already exists. Many homeowners go from first walkthrough to move-in within about a year, with septic upgrades and permit queues being the common variables.
On cost: a full ground-up NH ADU typically runs $275,000–$400,000+ depending on design. A garage conversion usually lands below a comparable new build because the foundation, framing, and roof already exist — but structural surprises, utility trenching, and septic work can narrow that gap, which is why the assessment phase comes first. For financing, a HELOC, home-equity loan, cash-out refinance, or construction loan are the usual tools, and some lenders will count projected ADU rent when qualifying you — ask a local lender who knows ADUs.
The garage is already sitting there. The law finally lets you use it — the only question left is whether yours is ready.
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