| Max size | 1,000 sq ftDetached: lesser of 50% or 1,000 |
|---|---|
| Min size | No town minimumbuild as small as code allows |
| Bedrooms | Up to 2 |
| Type | Attached + detachedDetached: CUP + 125% lot size |
| Parking | 1 spaceon top of main-house parking |
| Owner-occupancy | Requiredin either the ADU or the primary home |
| Short-term rental | Not stated |
| Utility connection | Tie into the existing electrical connection or choose a separate metersame statewide |
The July 2025 state law (RSA 674:71–73) overrides local ordinance language that conflicts with it — even language a town still has posted. Here's what that means in Merrimack:
Merrimack's ordinance still makes you go through a Planning Board hearing and win a discretionary Conditional Use Permit to build a detached ADU. Since July 1, 2025, state law entitles you to build one detached ADU with just a building permit — no hearing required. This is likely the single biggest gap between what Merrimack's posted ordinance says and what you're actually entitled to.
The town's ordinance requires your lot to be 25% larger than the normal minimum before you can even apply for a detached ADU — on top of requiring a hearing. That kind of acreage gate is exactly the type of restriction HB 577 was written to eliminate. If your lot meets the normal single-family minimum, state law says you should qualify for a detached ADU regardless of this extra 25% cushion.
Merrimack's ordinance flatly bans a separate utility connection for an attached ADU. State law specifically protects your right to a separate electrical meter/service for the ADU — the town cannot make you share utilities with the main house.
The town can require one extra parking space (that part is compliant), but it doesn't say — and can't require — that the space be on your own lot. State law gives you the choice of on-site or a dedicated off-site spot.
Merrimack's ordinance forces an attached ADU to share an internal connection and steers its access point to the side or rear. State law says the town can't dictate your entrance choice. Whether this specific 'functionally dependent' language crosses that line, versus being a permissible design-continuity rule, is a call for a land-use attorney.
If you already have a garage or barn you want to convert into a detached ADU, Merrimack's ordinance as written doesn't give that any different (easier) treatment than building a brand-new detached ADU from scratch — same CUP, same lot-area gate. State law protects your right to convert an existing structure even if it doesn't perfectly conform to today's setback rules.
Start with what state law now guarantees: since July 1, 2025, RSA 674:71-73 (as amended by HB 577) entitles you to one ADU — attached OR detached — by right wherever single-family homes are allowed, with no conditional use permit, no ADU-specific lot-size gate, and a protected right to separate electrical service. Merrimack's zoning ordinance (§2.02.1, District R - Permitted Uses), last revised October 27, 2016, has not been updated for that law and treats attached and detached ADUs very differently. An ATTACHED ADU is a by-right permitted use on any property with an owner-occupied single-family home: up to 1,000 sq ft, no more than two bedrooms, the owner must occupy one of the two units, one extra parking space, and a deed covenant barring condo conversion — recorded with the Hillsborough County Registry of Deeds. But the town's text also says it must be internally connected to the main house and may NOT have separate utilities, a separate garage, or a separate driveway — and on separate utilities, state law now overrides the town: a municipality may not deny an ADU separate electrical service. A DETACHED ADU is where the ordinance lags furthest: the town still says it requires a discretionary Conditional Use Permit from the Planning Board, must sit only in the side or rear yard, is capped at the lesser of 50% of the main house's size or 1,000 sq ft, and the lot must be at least 25% larger than the district's normal minimum lot size (roughly 1.15 to 2.9 acres depending on the sub-district). Under the 2025 state law, that CUP-hearing-plus-acreage-gate structure no longer controls: towns must permit one detached ADU by right, with no special hearing and no ADU-specific lot-size gate. The ordinance is also silent on whether your required extra parking space can be off-site (state law says that choice is yours) and doesn't spell out any accommodation for converting an existing detached garage into an ADU (state law protects that right even where the structure is nonconforming as to setbacks). Because the town's published text predates the state law by nearly a decade, confirm current practice with Merrimack's Planning & Community Development Department — and be ready to cite RSA 674:71-73. See conflicts[] for full citations.
Get a free personalized report for your address in Merrimack — including where the 2025 law works in your favor.
Get My Free Merrimack ADU Report →House Bill 577 (2025) amended RSA 674:72, effective July 1, 2025. Wherever Merrimack's ordinance is stricter than this floor, the state law controls. Read the full NH ADU law explainer →
Yes — one ADU, attached or detached, is allowed by right under the July 2025 state law. Merrimack's ordinance still says a detached ADU needs a Planning Board Conditional Use Permit, but state law now overrides that hearing requirement for your one by-right ADU. Under RSA 674:72 (as amended effective July 1, 2025), one accessory dwelling unit — attached or detached — is allowed by right on single-family lots statewide.
1,000 sq ft — Detached: lesser of 50% or 1,000. State law doesn't let any town cap ADU size below 750 sq ft.
Attached + detached (Detached: CUP + 125% lot size). Since July 1, 2025, state law allows detached ADUs by right statewide — town provisions that prohibited them are superseded.
1 space — on top of main-house parking. State law caps what any town can require at one additional space, located on- or off-site at the owner's choice.
Required — in either the ADU or the primary home. Where owner-occupancy is required, state law lets the owner choose which unit to live in.