Merrimack NH ADU Guide: What You Can Build in 2026

ADU law update (August 2026): New Hampshire keeps making accessory dwelling units easier. HB 1079, signed July 2, 2026 and effective August 31, 2026, closes a gap that kept tripping homeowners up - towns telling them to go get a zoning variance before converting an existing structure that already sits in a nonconforming spot. Under the new law, an ADU built within or attached to certain existing nonconforming structures no longer needs that variance. Stacked on top of HB 577 (in effect since July 1, 2025), the state has quietly handed Merrimack homeowners far more freedom than the town's own ADU paperwork would suggest. That gap is the whole story here.

If you've ever looked into adding an in-law suite, a garage apartment, or a backyard cottage in Merrimack, you probably ran into the town's old "special exception" ADU process - the one that requires a zoning board hearing, abutter notifications, and a promise that only family will ever live there. Here's the thing: state law has largely overtaken those rules.

HB 577 changed the rules statewide

Since July 1, 2025, HB 577 requires every New Hampshire town - Merrimack included - to allow one accessory dwelling unit by right on any lot where a single-family home is allowed. "By right" is the key phrase. It means:

• The unit can be attached or detached - your choice.
• The town can't force you through a special exception or conditional use permit just because it's an ADU.
• Towns can't cap an ADU below 750 square feet; the state default ceiling is 950 square feet unless the town allows more.
• The town can't add ADU-only setbacks or limit you to one bedroom, and can't pile on extra frontage, lot-size, or design hurdles beyond what a single-family home already faces.

Towns are still allowed to require one off-street parking space and owner-occupancy of one of the two units. But that's about the extent of it.

What Merrimack's own ordinance still says - and why it's outdated

Merrimack's ADU rules (Section 2.02.1(B)(2)) were written well before HB 577, and the town's special-exception application still reflects the old world. On paper it asks that an ADU:

• Go through a Zoning Board special exception with abutter notification,
• Be contained within or attached to the main house and connected internally - i.e., no true detached unit,
• Contain no more than one bedroom,
• Not exceed 1,000 square feet or 50% of the main home's heated living space, whichever is smaller,
• Be occupied by family members only, recorded as a deed covenant,
• Have no separate utilities, garage, or driveway.

Read that list next to HB 577 and you can see the collision. The by-right requirement overrides the special-exception hearing. The detached-ADU right overrides "internally connected only." The state's floor and bedroom protections override the one-bedroom cap. And a family-only occupancy restriction is not something the state law contemplates for a by-right ADU - you're allowed to rent it. Where a state statute and a local ordinance disagree, the state law wins.

The practical catch: the person at Merrimack's permit counter may still be working from the old ordinance text, because towns don't always rewrite their books the moment Concord acts. So bring the statute (RSA 674:71-72), ask specifically how Merrimack is handling by-right detached ADUs and rentals post-HB 577, and get the answer in writing before you spend a dollar.

Do the numbers work in Merrimack?

They work well - arguably better than most Southern NH towns, because Merrimack rents are strong. In 2026, one-bedrooms here average around $2,200/month and two-bedrooms run $2,400-$2,600. A finished ADU realistically brings in $1,700-$2,300/month depending on size and finish.

On the cost side, converting existing space - a garage or basement - generally runs $80K-$150K, since you reuse the foundation, walls, and roof (and HB 1079 makes that path cleaner if your garage sits in a nonconforming spot). A brand-new detached cottage lands around $150K-$300K+, with New Hampshire build costs running about $150-$400 per square foot in 2026. At $2,000+/month in rent, a lot of Merrimack ADUs cover their own financing and then some - and they add real, appraisable value whether you rent to a tenant or house a parent or an adult kid. If you're weighing where to buy or build, our Merrimack NH real estate page is a good place to start, and our guide to homes for sale in Merrimack breaks down the neighborhoods where ADU-friendly lots tend to be.

Quick answers: Merrimack NH ADUs

Can I build a detached ADU in Merrimack, NH? Yes. Since HB 577 took effect July 1, 2025, one detached or attached ADU is allowed by right on any lot that permits a single-family home - even though Merrimack's older ordinance language leaned toward internally-connected units only.

Does Merrimack still require a special exception for an ADU? For a by-right ADU, no. HB 577 removed the town's ability to force ADUs through that process, though the town's paperwork may not reflect it yet - confirm in writing.

Can I rent my Merrimack ADU to a non-family member? Under HB 577, a by-right ADU isn't subject to the old family-only restriction. This is the biggest change from the town's old rules, so verify it directly with Merrimack Planning.

How big can a Merrimack ADU be? State law protects a floor of 750 square feet and a default ceiling of 950 square feet unless the town allows larger.

Thinking about it? Let's run your specific lot

Every lot is different - setbacks, existing structures, sewer, and that pre-2025 conversion angle can swing a project from "maybe" to "absolutely." The Phinney Team will give you a free property feasibility read: tell us your address and what you're picturing, and we'll tell you what's realistic. Want to scope it yourself first? Grab our ADU Feasibility Checklist and walk through it in ten minutes. Reach out anytime at teamphinney.com - Aaron Phinney, The Phinney Team at Keller Williams Realty Metropolitan, Bedford, NH.

This guide is general information, not legal advice. Merrimack's zoning ordinance is periodically updated, and HB 577's interaction with local rules is still settling in. Always confirm current requirements with the Town of Merrimack Planning & Zoning Department before you design or build.