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

By Aaron Phinney, The Phinney Team at Keller Williams Realty Metropolitan

If you own a home in Concord and you've been eyeing that detached garage, the space over it, or the corner of your lot wondering "could I put a rental there?" — the answer changed in a big way last summer, and most homeowners still don't realize it.

New Hampshire's ADU law, HB 577, took effect July 1, 2025. A year in, it's quietly rewritten the rules for accessory dwelling units across the state — and here in Concord, the city's own zoning ordinance hasn't fully caught up yet. That gap is exactly where the opportunity lives. My brother Andrew and I own rentals here in Concord, so this isn't theory for us. Here's the plain-English version.

What HB 577 gives every Concord 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. You no longer need the stars to align for a backyard cottage or a converted-garage unit. The state requires municipalities to permit one detached ADU.
• Size floor of 750 sq ft. A town can't cap your ADU below 750 square feet, and the default ceiling is 950 square feet unless the city allows larger.
• No owner-occupancy mandate. This is the sleeper. HB 577 removed a town's ability to force you to live in either unit. You can rent both.
• Parking stays reasonable. A city can require at most one extra space for the ADU, and you can put it on-site or at a legal off-site spot.
• Same rules as a single-family home. A town can't pile on extra setbacks, lot-size, or design hurdles just because there's an ADU on the property.

Where Concord's ordinance is behind

Here's the part to pay attention to. Concord's zoning ordinance (Article 28-5-52) still reads like the old world — it describes ADUs as attached or within the main house, caps them at 600–750 sq ft, requires owner-occupancy, and routes them through a special exception at the Zoning Board. Several of those provisions now conflict with HB 577 and are superseded by state law: detached units are allowed, the size floor is 750 (not a ceiling), and the owner-occupancy requirement is no longer enforceable.

Concord is in the middle of a broader zoning rewrite — the city passed interim amendments in 2025 and has a full master-plan-driven overhaul on the horizon — so the ordinance language is a moving target. Don't price a project off the old book. Confirm the current standards with the Concord Planning Division before you draw plans, and lean on the state law where the two disagree.

Does the math work in Concord?

Concord's rental market is strong. One-bedroom rents are clustering around $1,500–$1,900/month in 2026, and a well-finished ADU can land in the $1,500–$2,200 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 ~$120K and rent it at $1,700/month — that's about $20,400 a year in gross rent, or a mid-teens gross yield on the build. New detached at $220K renting $1,900 pencils closer to 10%. Beyond cash flow, an income-producing ADU adds real resale value and gives you options — aging parents, an adult kid, or a straight rental. If you want to see how those numbers translate into a bigger portfolio, our Bedford NH ADU guide walks through the same framework for a neighboring market.

Your next step

The single biggest mistake I see is homeowners guessing at feasibility instead of checking. Setbacks, septic capacity (Concord ADUs must meet NH RSA 485-A standards), lot shape, and utility runs decide whether a project is a slam dunk or a headache.

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 Concord lot, and what it could be worth. Reach The Phinney Team at teamphinney.com or (603) 568-3399.

We know Concord because we invest here ourselves.

This guide is general information, not legal advice. ADU rules are changing and Concord is actively revising its zoning code — always confirm current requirements with the Concord Planning Division before starting a project.

Concord NH ADU — Frequently Asked Questions

Can I build a detached ADU in Concord, NH?
Yes. Under New Hampshire's HB 577 (effective July 1, 2025), a detached accessory dwelling unit is allowed by right on any lot where a single-family home is permitted. This supersedes older Concord ordinance language that referenced attached units only.

How big can a Concord ADU be?
State law prevents towns from capping an ADU below 750 square feet, with a default ceiling of 950 square feet unless the city authorizes something larger.

Do I have to live in the home to have an ADU in Concord?
No. HB 577 removed municipalities' authority to require owner-occupancy, so you can rent both the main home and the ADU. Concord's older ordinance language requiring owner-occupancy is no longer enforceable.

How much does an ADU cost to build in Concord, 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.