Building an ADU in Concord, NH: The 2026 Deep Dive

Yes — Concord allows one accessory dwelling unit (ADU), attached or detached, by right on any lot where single-family homes are permitted. The city rewrote its ADU ordinance in an October 2025 amendment that closely tracks New Hampshire's new state law — RSA 674:72, amended by House Bill 577 (2025 Chapter 197), effective July 1, 2025 — and in a few places is more generous than the state requires.

That makes the capital city one of the more homeowner-friendly ADU towns in Merrimack County on paper. But Concord is also a case study in why you should read the current rules, not the first PDF a search engine hands you: the city's old 2017-era ADU section still circulates in stale caches and old handouts, and nearly every number in it is now obsolete. This guide is based on the city's posted ordinance documents, not a line-by-line confirmation with city staff — so treat it as your map, and confirm the specifics with Concord's planning and code administration offices before you draw plans.

What did Concord's October 2025 ADU amendment change?

The amendment rewrites the city's ADU section (§28-5-52) essentially from scratch. The headline changes, as we read them:

• By right, everywhere single-family homes are: one ADU, attached or detached, is allowed as a matter of right in every zoning district that permits single-family detached dwellings by right. No special exception, no variance, no conditional-use permit, no site plan review to establish the ADU itself.
• A bigger size allowance: the cap rises from the old 750 sq ft to a 950 sq ft base, expandable in some cases (details below), and the old 600 sq ft minimum disappears entirely.
• Sane parking: one space for the main house plus one for the ADU, and the ADU's space can be at a legally dedicated off-site location.
• No more special design review: the old rule that forced Planning Board Architectural Design Review specifically because a project included an ADU was struck. State law backs this up — an ADU can't be held to stricter aesthetic standards than an ordinary single-family home.
• Entrance is your choice: the old mandate for a common wall and interior connecting door is gone; an attached ADU can have an independent entrance or shared access, whichever you prefer.

One honest caveat: the published amendment document doesn't show a filled-in formal adoption date. The practical impact is limited, because RSA 674:72 is statewide law that applies regardless of whether a town's own code text has caught up — but before you rely on the amendment's exact local numbers, ask the city's Code Administrator to confirm it's in force.

How big can an ADU be in Concord?

Under the October 2025 amendment as we read it:

• Base cap: 950 sq ft of floor area.
• It can go higher — up to whichever is less of 50% of the principal dwelling's floor area or 1,250 sq ft. On a 2,000 sq ft house, that means up to 1,000 sq ft; on a bigger home, up to the 1,250 ceiling.
• No minimum size. The amended section states there is no minimum floor-area requirement, so a compact studio ADU is fine.

For context, state law lets a town cap ADU size but never below 750 sq ft — Concord's 950-and-up framework sits comfortably above that floor. There is no state minimum size; Concord's old 600 sq ft minimum was a local choice, and it's gone.

Can you build a detached ADU or convert a garage in Concord?

Yes on both counts. Detached ADUs are allowed by right, same as attached ones. And Concord's amendment is notably friendly to conversions: an existing detached structure — say, a garage that doesn't meet today's setbacks, lot coverage, or height rules — can still be converted to an ADU, as long as the conversion doesn't make the existing nonconformity any worse. That mirrors the state-law protection for garage conversions and is one of the cheapest paths to a legal second unit, since the shell already exists.

New-construction detached ADUs still have to respect the district's normal dimensional standards — setbacks, lot coverage, height — like any accessory structure. Those tables are district-specific, so pull your zone's numbers with the planning office before siting the building.

What about parking, owner-occupancy, and renting it out?

• Parking: one space for the principal dwelling plus one for the ADU, and the ADU's space may be off-site at a legally dedicated location. The old formula — two spaces plus one more per ADU bedroom — is superseded. State law caps what any town can require at one additional space, owner's choice of on-site or off-site.
• Owner-occupancy: the old ordinance required documented proof that the owner lived in one of the units, re-verified every three years. That requirement doesn't appear anywhere in the rewritten section — it looks like the city dropped it. Because state law allows (but doesn't require) towns to impose owner-occupancy, this was Concord's own call, and it's exactly the kind of detail worth double-checking with the Code Administrator before you build a rental plan around it.
• Bedrooms: the old two-bedroom cap doesn't appear in the amended text either. State law separately bars towns from limiting an ADU to one bedroom.
• Ownership structure: you can't condo off the ADU or sell it separately from the main house — one lot, one ownership. The main house and ADU each get their own address and mail receptacle.
• Short-term rentals: the ADU section is silent on STRs, old and new. If Airbnb-style use is part of your plan, ask the city directly.

Why do the old 750-square-foot rules still show up online?

Because the pre-amendment text — 750 sq ft cap, 600 sq ft minimum, two-plus parking spaces, mandatory shared interior door, owner-occupancy proof — was captured by code-hosting sites and city handouts for years, and stale copies don't delete themselves. If you find those numbers in an old PDF or hear them quoted over the counter, they're superseded twice over: once by Concord's own October 2025 amendment, and independently by state law, which controls even where local code text lags. Our town guides exist precisely to flag this gap, and Concord's town page tracks it. Neighboring towns like Bow and Hopkinton have their own versions of the same story.

What does the permitting process look like in Concord?

Concord runs permitting through an online self-service portal. Expect a normal building permit application, then the standard inspection gauntlet before a Certificate of Occupancy: building, mechanical, electrical, and plumbing inspections; a water/sewer connection (or private-system test results); an air-tightness (blower-door) test; a fire life-safety inspection; and payment of the city's invoiced impact fees. The exact impact-fee amount isn't spelled out in the ADU ordinance itself — get the current schedule from the planning or finance office early, so it's in your budget and not a surprise at C.O. time.

What does a Concord ADU cost, and is it worth it?

A typical New Hampshire ADU build runs $275,000–$400,000+ depending on design; call it a $300k project for planning purposes. Set against Concord rents — or against assisted living for an aging parent, which commonly cited industry surveys put at $70,000–$100,000+ per year in New Hampshire — the math can work out faster than most homeowners expect. Financing usually comes from home equity (a HELOC, home-equity loan, or cash-out refinance) or a construction loan, and some lenders will count projected ADU rent when qualifying you — ask a local lender who knows ADUs.

Concord did the hard part for you in October 2025; your job is just to confirm the details at city hall and build.