ADU Guide · Verified 2026
Building an ADU in New Hampshire
New Hampshire was one of the earliest states to require ADUs, back in 2017, but that original law only covered attached units. A 2025 amendment finally required municipalities to permit detached ADUs too. Unlike most states in this research, New Hampshire still allows municipalities to require owner occupancy.
Quick Facts
Statewide floors set by RSA 674:71-73 (2017); detached units added by HB 577 (2025). Your city may be more generous but cannot go below them.
The Three Types of ADU
One of the earliest ADU laws, but with a real gap for years
New Hampshire's original RSA 674:71-73 from 2017 predates most of the other state laws in this research. But that original law covered only attached or internal ADUs, meaning a detached backyard cottage had no statewide protection at all until HB 577 closed that gap in 2025, a genuinely long gap between the two.
A real exception to the no-occupancy trend
Unlike California, Colorado, Arkansas, or Oregon, New Hampshire's statute expressly permits municipalities to require owner occupancy of the primary home, the ADU, or both, and to impose limited parking requirements, a real, deliberate choice to leave more local control than most other states in this research.
What municipalities still cannot do
Even with the occupancy and parking exceptions, RSA 674:71-73 bars municipalities from using indirect zoning tools, special-use permits, discretionary or aesthetic review, or dimensional standards stricter than the primary home's, to effectively reimpose the barriers the legislature specifically removed.
What people get wrong
The most common mistake is assuming New Hampshire's 2017 law always covered detached ADUs. It didn't, that protection is genuinely new, added by HB 577 in 2025, eight years after the original attached-unit right.
From Idea to Move-In: The Timeline
Who Administers It
Building It Yourself? Here's What You'll Need
Planning an ADU? Tools homeowners use in the design and layout phase:
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Check Your Local Rules
Your city may allow more than the state floor
State law sets the minimum; many New Hampshire cities and counties are more generous. Browse the full directory of 10 counties and every city in New Hampshire for local rules, fees, and building department contact details.
Browse the full New Hampshire directory →Not sure if your project needs a building permit, a zoning approval, or both? See our guide to building permits vs. zoning approval.
Official Sources
ADU law changes frequently and local ordinances add detail. This guide is general information, not legal advice, always confirm with your local building department. Reviewed by the StateDataIndex Editorial Team · Updated September 2026.