The Nashua NH ADU Guide: What HB 577 Lets You Build in 2026

Quick answer: As of HB 577 (effective July 1, 2025), any Nashua homeowner whose lot allows a single-family home can add one accessory dwelling unit — attached or detached — by right, with no trip to the Zoning Board. Nashua updated its own code in December 2025 (ordinance O-25-076) to match. A local ADU can run up to 980 square feet (up to 1,200 if you convert space inside an existing structure), needs one extra parking space, and you must live in one of the two units — your choice which. Always confirm current standards with the Nashua Planning Department.

If you own a home in Nashua and you've looked at your detached garage, your walk-out basement, or the back corner of your lot and thought "could I put a rental there?" — the answer changed in a big way, and Nashua just rewrote its own code to match. My brother Andrew and I own a rental here in Nashua, so this isn't theory for us. Here's the plain-English version.

ADU law update: what changed in 2026

New Hampshire's ADU law, HB 577, took effect July 1, 2025. It made one attached or detached ADU legal by right statewide. In December 2025, Nashua's Board of Aldermen passed ordinance O-25-076 to bring the city's land-use code into line with the state. And in July 2026, the Legislature added HB 1079, a follow-up law aimed squarely at making it easier to convert existing structures — garages, barns, carriage houses — into ADUs. The through-line across all three: the state wants these units built, and it keeps stripping away the red tape that used to stop them.

What HB 577 gives every Nashua homeowner

Under state law, if your lot allows a single-family home, you now have the right to one accessory dwelling unit — attached OR detached. A few things worth underlining:

• Detached is now by right. A backyard cottage or a converted detached garage no longer needs a special exception. The state requires Nashua to permit one detached ADU on eligible lots.
• No more Zoning Board trip. ADUs are now permitted by right in every district that allows single-family homes — no conditional use permit, no special exception.
• Size floor of 750 sq ft. A town can't cap your ADU below 750 square feet. Nashua is actually more generous than the state's 950 default: local code allows up to 980 square feet, and up to 1,200 square feet if the unit is built inside an existing structure without expanding its footprint.
• Convert an old garage even if it's "too close." This is the sleeper. HB 577 requires the city to let you convert an existing structure — including a detached garage — into an ADU even if it violates today's setback or lot-coverage rules, as long as the structure existed on or before July 1, 2025. In older, tightly-platted Nashua neighborhoods — the Tree Streets, French Hill, Crown Hill — that unlocks a lot of otherwise "nonconforming" garages and carriage houses.
• Parking stays reasonable. Nashua requires one extra off-street space for the ADU, and the state caps it there.

Where Nashua's rules still apply

Nashua kept a few local requirements that HB 577 allows, so know these before you draw plans. The ADU must stay "clearly incidental and subordinate" to the main house and generally match its architectural style, with no new street-facing entrance or curb cut. You'll record a covenant with the City and the Hillsborough County Registry of Deeds noting the ADU, and the unit can't be sold separately or condo-converted.

The one that surprises people: Nashua still requires the owner to live in one of the two units. HB 577 let towns keep an owner-occupancy rule for a single unit — it just took away their power to dictate which one. So you can live in the main house and rent the ADU, or live in the ADU and rent the house, but you can't rent both. (This is different from some neighboring towns, and it's exactly the kind of detail worth confirming with the Nashua Planning Department, since HB 1079 and future code tweaks keep nudging these rules.)

Does the math work in Nashua?

Nashua's rental market is one of the strongest in the state — it's New Hampshire's second-largest city, right on the Massachusetts line, with steady commuter demand. One-bedroom rents are running roughly $1,700–$2,000/month in 2026, and a well-finished ADU can land in the $1,600–$2,100 range depending on size and finish.

On the cost side, a garage or basement conversion typically runs $80K–$150K, while a new detached unit runs $150K–$300K+. Run a simple example: convert a garage for about $120K and rent it at $1,900/month — that's roughly $22,800 a year in gross rent, a high-teens gross yield on the build. A new detached unit at $220K renting for $2,000 pencils closer to 11%. Beyond monthly cash flow, an income-producing ADU adds real resale value and gives you options — aging parents, an adult kid, or a straight rental. Our Concord NH ADU guide and Bedford NH ADU guide walk through the same framework for neighboring markets.

Your next step

The single biggest mistake I see is homeowners guessing at feasibility instead of checking. Setbacks, sewer or septic capacity, lot shape, and utility runs decide whether a project is a slam dunk or a headache. In Nashua, most lots are on city sewer, which usually makes an ADU easier than in septic-dependent towns — but capacity and connection costs still vary block to block.

Grab our free ADU Feasibility Checklist — it walks you through the exact questions to answer before you spend a dollar. Or send me your address and I'll give you a free, no-obligation feasibility read on what HB 577 realistically lets you build on your specific Nashua lot, and what it could be worth. Reach The Phinney Team at teamphinney.com or (603) 568-3399.

We know Nashua because we invest here ourselves.

This guide is general information, not legal advice. ADU rules are changing — HB 1079 took effect in 2026 and Nashua continues to refine its land-use code — so always confirm current requirements with the Nashua Planning Department before starting a project.

Nashua NH ADU — Frequently Asked Questions

Can I build a detached ADU in Nashua, NH?
Yes. Under New Hampshire's HB 577 (effective July 1, 2025) and Nashua's matching ordinance O-25-076, a detached accessory dwelling unit is allowed by right on any lot where a single-family home is permitted — no special exception or Zoning Board approval required.

How big can a Nashua ADU be?
Nashua allows an ADU up to 980 square feet, or up to 1,200 square feet if it's created inside an existing structure without expanding the footprint. State law prevents any town from capping an ADU below 750 square feet.

Do I have to live on the property to have an ADU in Nashua?
Yes — Nashua requires the owner to occupy one of the two units, but you choose which. You can live in the main home and rent the ADU, or live in the ADU and rent the main home. Under HB 577 the city can no longer dictate which unit you occupy.

How much does an ADU cost to build in Nashua, NH?
A garage or basement conversion typically runs $80,000–$150,000, and a new detached unit runs $150,000–$300,000 or more, depending on size, site work, and finish.