Build an ADU in Alton.
New Hampshire's HB 577 lets you add one accessory dwelling unit on your single-family lot in Alton — 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 Alton zoning ordinance · source · 2026-07-06
- Front
- 25 ft
- Side
- 10 ft
- Rear
- 10 ft
- Max coverage
- —
- Min lot
- 21,780 sf (~0.50 ac)
Primary single-family residential district. Min lot for a single-family dwelling (Sec 433.A): 1/2 acre (21,780 sf) WITH municipal water; 1 acre (43,560 sf) WITHOUT municipal water — value shown is the with-water minimum.
- Front
- 25 ft
- Side
- 10 ft
- Rear
- 10 ft
- Max coverage
- —
- Min lot
- 30,000 sf (~0.69 ac)
Min lot area 30,000 sf (0.689 ac), one single-family dwelling per lot (Sec 412.A). Frontage: lakefront lots 150 ft at the shoreline (lots after 14 Mar 1995) plus 50 ft at the street ROW (lots after 13 Mar 2018; 30 ft before); all other lots 75 ft (Sec 412.B).
- Front
- 25 ft
- Side
- 10 ft
- Rear
- 10 ft
- Max coverage
- —
- Min lot
- 15,000 sf (~0.34 ac)
Mixed residential/commercial district; only one residential use per lot. Min lot for single-family dwelling (Sec 443.A.1): 15,000 sf (0.344 ac) WITH municipal water; 30,000 sf (0.69 ac) WITHOUT — value shown is the with-water minimum.
- Front
- 25 ft
- Side
- 10 ft
- Rear
- 10 ft
- Max coverage
- —
- Min lot
- 43,560 sf (~1.00 ac)
Min lot area 1 acre (43,560 sf) for a single-family dwelling, one per lot; duplex/townhouse/multi-family 1 acre per dwelling unit; all other uses 1 acre (Sec 463.A). Frontage 150 ft (Sec 463.B). Setbacks town-wide (Sec 327): 25 ft from ROW, 10 ft from all other property lines.
- Front
- 25 ft
- Side
- 20 ft
- Rear
- 20 ft
- Max coverage
- —
- Min lot
- 87,120 sf (~2.00 ac)
Most-restrictive rural district. Min lot area 2 acres (87,120 sf) for a single-family dwelling, one per lot; multi-family/townhouse 2 acres per dwelling unit (Sec 452.A). On lots >45 acres a second SF dwelling is allowed if sited for future subdivision (Sec 452.A note).
- Front
- 25 ft
- Side
- 10 ft
- Rear
- 10 ft
- Max coverage
- —
- Min lot
- —
Non-residential recreation/transient-service district outside the urban area (high-density recreational uses).
Form-based and downtown districts may use build-to lines instead of fixed setbacks (shown as “—”); verify your parcel's district before you build.
Alton 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 Alton (Belknap County). A detached ADU can be up to 950 sf.
In Alton's R zone, ADUs follow the single-family setbacks: 25 ft front, 10 ft side, and 10 ft rear. Alton has 6 zoning districts in total — see the full table below. Confirm your exact zone with the town.