| Max size | 40% of the main houseconfirmed with the town |
|---|---|
| Min size | no minimum SF, however town can't restrict you…confirmed with the town |
| Bedrooms | Up to 2confirmed with the town |
| Type | Attached + detachedSide or rear yard only |
| Parking | 1 spot onsiteconfirmed 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 Londonderry:
Londonderry doesn't cap ADU size in square feet — it caps it at 40% of the principal home's living area, with no stated floor. For any home under roughly 1,875 sq ft of living area, that formula produces a cap below the state's 750 sq ft minimum, which state law doesn't allow. If your house is smaller, you're entitled to build up to 750 sq ft regardless of what 40% of your home's size works out to.
The town can require one extra parking space, but it can't dictate that it sit in your rear or side yard specifically — state law gives you the choice of putting it on-site or at a dedicated off-site spot.
Londonderry's ordinance flatly bans a street-facing second door for an attached ADU. State law says the choice of entrance — shared or separate, and where it goes — belongs to the homeowner, not the town.
If you want to convert an existing detached garage into an ADU but that garage already sits closer to the property line than today's setback rules allow, Londonderry's ordinance as written doesn't say you're exempt — but state law says you are. This is a gap in the town's text rather than an outright ban, so get an attorney's read before treating it as settled.
CONFIRMED WITH THE TOWN (August 19,2026) — ADUs allowed: 1; maximum size 40% of the main house; minimum size no minimum SF, however town can't restrict you below 750 SF; up to 2 bedrooms; impact fees: No; parking: 1 spot onsite; detached-unit distance rule: No; setbacks: Residnential AR-1 40’ Front, 15’ Side and rear. Town contact: Nick Codner - Cheif Building Inspector 603-432-1100 Ext. 161.
Not at this time
/Dylan called Wednesday the 29th morning still waiting for a call back to verify final details will update last of the information then. Pat Confirmed August 19, 2026
Start with the state law, because it sets the floor: since July 1, 2025, RSA 674:71-73 (as amended by HB 577) entitles every New Hampshire homeowner to one accessory dwelling unit — attached or detached — by right, wherever single-family homes are allowed, with no special exception or conditional use permit. Londonderry's own rules live in Zoning Ordinance §5.18 'Accessory Dwellings' (page 141), last amended November 30, 2020, and the town was already ahead of its time on the single biggest issue: it explicitly allows a DETACHED ADU, including converting an existing free-standing garage, sited in the side or rear yard — no special exception or conditional use permit required, just a by-right permitted accessory use once the listed conditions are met. Owner-occupancy is required (in either unit, the town doesn't say which — a requirement state law permits), the ADU is capped at 2 bedrooms (also permitted — state law only bars a 1-bedroom cap), and the town does NOT let you condo-ize or separately convey the ADU (a restriction state law allows). Where the town's 2020 ordinance no longer holds up against the 2025 state floor — in each case the ordinance still says one thing, but state law now overrides it: (1) size is capped at 40% of the principal home's living area with no flat square-footage floor — for any home under about 1,875 sq ft, that formula caps the ADU below the state's 750 sq ft minimum, and no town cap may go below 750 sq ft; (2) the required parking space must sit in your rear or side yard specifically — state law says you choose between on-site and a legally dedicated off-site location; (3) an attached ADU's exterior entry is flatly barred from facing the street as a second door — state law says the choice of a shared vs. separate entrance is the homeowner's; (4) the ordinance states no exemption for converting an existing, already-nonconforming detached structure (like a garage sitting inside today's setback) — state law requires that conversion be allowed. See conflicts[] for full citations. Because the ordinance text predates the 2025 state law, confirm current practice with the town's planning and building offices — and be ready to cite RSA 674:71-73.
Get a free personalized report for your address in Londonderry — including where the 2025 law works in your favor.
Get My Free Londonderry ADU Report →House Bill 577 (2025) amended RSA 674:72, effective July 1, 2025. Wherever Londonderry's ordinance is stricter than this floor, the state law controls. Read the full NH ADU law explainer →
Yes — Londonderry allows one accessory dwelling unit (attached or detached, including garage conversions) by right on any property with an owner-occupied single-family home. 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.
40% of the main house — confirmed with the town. State law doesn't let any town cap ADU size below 750 sq ft.
Attached + detached (Side or rear yard only). Since July 1, 2025, state law allows detached ADUs by right statewide — town provisions that prohibited them are superseded.
1 spot onsite — 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.