| Max size | 950 sq ftconfirmed with the town |
|---|---|
| Min size | 750 sq ftconfirmed with the town |
| Bedrooms | Up to 2confirmed with the town |
| Type | Attached or detachedincl. detached garage conversions |
| Parking | 1 spot on or off siteconfirmed with the town |
| 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 Derry:
Derry's own definitions section was updated in 2025 to say you CAN build the ADU at the same time as a brand-new house, but the operative permit-conditions section (165-25) still has old language implying the house must already exist. That's a contradiction inside Derry's own ordinance. State law resolves it in your favor: you don't have to wait and retrofit.
The town can require one extra parking space, but the choice of where it goes — on your lot or a dedicated off-site spot — is yours by state law, not the town's.
Derry softened this clause in 2025 but didn't fully fix it — it still reads as if an interior shared door, when present, must function as your PRIMARY entrance. State law and NH Housing's own guide say the choice of a shared vs. separate entrance is yours, period.
Derry can only hold your ADU to the same look-and-feel rules it holds a regular house to — not stricter ones. Whether the 'maintain continuity' clause crosses that line is an attorney call.
CONFIRMED WITH THE TOWN (July 29,2026) — maximum size 950 sq ft; minimum size 750 sq ft; up to 2 bedrooms; parking: 1 spot on or off site. Town contact: test information.
Dylan called Wednesday the 29th morning still waiting for a call back to verify final details will update last of the information then.
Derry's ADU ordinance (Zoning §165-25) was substantially rewritten by the Town Council on November 18, 2025, effective December 19, 2025 — AFTER the July 1, 2025 state law (RSA 674:71-73 as amended by HB 577) took effect, and it shows: the town closed its biggest gap by deleting the old ban on converting a detached garage or other free-standing structure into an ADU, and it now says explicitly that a lot may have one ADU, attached OR detached. Size limits were also updated to match the state floor/ceiling exactly: no less than 750 sq ft, no more than 950 sq ft of living space. Owner-occupancy is required (in either unit, town doesn't say which — compliant), max two bedrooms, one extra off-street parking space, and the ADU counts as an exception to Derry's one-building-per-lot rule. Where Derry still lags the new state floor: (1) the town's operative permit clause still says an ADU may only be created 'in an existing single-family detached dwelling,' even though the town's own updated definitions section says ADUs 'may be constructed at the same time as the principal dwelling unit' — an internal contradiction that state law resolves in your favor; (2) the town doesn't say you can put the required parking space off-site — state law gives you that choice; (3) the interior-door language still implies a shared door must serve as your PRIMARY entrance when present — state law says the choice of shared vs. separate entrance is yours; (4) a 'design continuity' aesthetic clause could be read as more restrictive than what's allowed for a regular house — an attorney call. See conflicts[] for full citations and resolved_by_town[] for what Derry already fixed.
Primary source: eCode360 (General Code) — §165-25 amended 10-17-2005
Get a free personalized report for your address in Derry — including where the 2025 law works in your favor.
Get My Free Derry ADU Report →House Bill 577 (2025) amended RSA 674:72, effective July 1, 2025. Wherever Derry's ordinance is stricter than this floor, the state law controls. Read the full NH ADU law explainer →
Yes — Derry allows one accessory dwelling unit (attached or detached) by right in its residential districts. 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.
950 sq ft — confirmed with the town. State law doesn't let any town cap ADU size below 750 sq ft.
Attached or detached (incl. detached garage conversions). Since July 1, 2025, state law allows detached ADUs by right statewide — town provisions that prohibited them are superseded.
1 spot on or off site — confirmed with the town. 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.