Build an ADU in Thornton.
New Hampshire's HB 577 lets you add one accessory dwelling unit on your single-family lot in Thornton — by-right. A detached ADU can be up to 950 sf, and ADU setbacks match your lot's single-family setbacks.
6 districts
Each district's single-family setbacks — which your ADU follows. The primary residential district is highlighted.
Verified against the official Thornton zoning ordinance · source · 2026-07-07
- Front
- 25 ft
- Side
- 15 ft
- Rear
- 15 ft
- Max coverage
- —
- Min lot
- 43,560 sf (~1.00 ac)
Predominant district: 'includes all land, roads and water areas, excluding the White Mountain National Forest, outside those areas defined as General Residence Zone, Commercial Zone, Industrial Zones, and Recreational Zone West' (Art III.B.1).
- Front
- 25 ft
- Side
- 15 ft
- Rear
- 15 ft
- Max coverage
- —
- Min lot
- 43,560 sf (~1.00 ac)
Dimensional requirements identical to Rural Residence. Setbacks (Art VI.A.2 / Art VII): 25 ft from any state/local/private highway/road right-of-way ('front', measured from ROW not centerline), 15 ft from every other boundary line ('side'/'rear').
- Front
- 25 ft
- Side
- 15 ft
- Rear
- 15 ft
- Max coverage
- —
- Min lot
- 43,560 sf (~1.00 ac)
Recreational Zone West (Art III.B.3). Residential development governed by Art VI.C: basic density 1 acre per dwelling (43,560 sf); individual lot sizes within planned development tracts may be smaller than 1 acre but overall dedicated density must remain 1 acre/dwelling.
- Front
- —
- Side
- —
- Rear
- —
- Max coverage
- —
- Min lot
- —
Commercial Zone (Art III.B.4). Art VII lists 'NS' (Not Specified) for lot size, frontage, setbacks, open space and height in the Commercial column: commercial development is exempt from specific minimum-lot-size requirements and is instead subject to Planning Board Site Plan…
- Front
- —
- Side
- —
- Rear
- —
- Max coverage
- —
- Min lot
- —
Industrial Zone I-1 (Art III.B.5.1). Art VII lists 'NS' (Not Specified) for lot size, frontage, setbacks and height in the Industrial columns. Residential development in Industrial Zone I & II requires 1 acre per dwelling unit (43,560 sf) and follows the Conventional/Cluster…
- Front
- —
- Side
- —
- Rear
- —
- Max coverage
- —
- Min lot
- —
Industrial Zone I-2 (Art III.B.5.2). Art VII lists 'NS' (Not Specified) for lot size, frontage, setbacks and height (Industrial II column). Same treatment as Industrial I-1: industrial/commercial development exempt from minimum-lot-size and subject to Site Plan Review (Art…
Form-based and downtown districts may use build-to lines instead of fixed setbacks (shown as “—”); verify your parcel's district before you build.
Thornton ADU FAQ
Yes. Under NH RSA 674:71-73 (HB 577), one accessory dwelling unit is allowed by-right on single-family lots statewide, including Thornton (Grafton County). A detached ADU can be up to 950 sf.
In Thornton's RR zone, ADUs follow the single-family setbacks: 25 ft front, 15 ft side, and 15 ft rear. Thornton has 6 zoning districts in total — see the full table below. Confirm your exact zone with the town.