Build an ADU in New Castle.
New Hampshire's HB 577 lets you add one accessory dwelling unit on your single-family lot in New Castle — 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 New Castle zoning ordinance · source · 2026-07-07
- Front
- 20 ft
- Side
- 15 ft
- Rear
- 15 ft
- Max coverage
- 20% impervious
- Min lot
- 20,000 sf (~0.46 ac)
Principal residential district: the central densely settled village area; permits single- and two-family dwellings (Sec 4.1.2). Frontage 100 ft Max height 2.5 stories / 32 ft; min roof pitch 1:3. Coverage is tiered by LOT SIZE, same in all districts (Sec 4.2.1.3, impervious…
- Front
- 20 ft
- Side
- 15 ft
- Rear
- 15 ft
- Max coverage
- 20% impervious
- Min lot
- 43,560 sf (~1.00 ac)
Single- and two-family residential at lower density than R-1 (east/west of Wentworth Road areas, Sec 3.1.2). Frontage 100 ft Max height 2.5 stories / 32 ft Same lot-size-tiered impervious coverage schedule and Maximum Building Area formula as R-1 (Table 1 rows H-I: 'Same as…
- Front
- 40 ft
- Side
- 20 ft
- Rear
- 20 ft
- Max coverage
- 20% impervious
- Min lot
- 87,120 sf (~2.00 ac)
Primarily single-family, lowest density (near Great Island Common and Fort Stark, plus most islands; Sec 3.1.3). Max 1 dwelling unit per building (Table 1 row J). Frontage 100 ft Max height 2.5 stories / 32 ft Same tiered impervious coverage schedule and Maximum Building Area…
- Front
- 20 ft
- Side
- 10 ft
- Rear
- 10 ft
- Max coverage
- 20% impervious
- Min lot
- 43,560 sf (~1.00 ac)
Wentworth Road corridor at the Rye town line; clustered residential and resort/hotel uses per Sec 6.1 special regulations. Frontage 100 ft Clustered development follows Sec 6.1 instead (e.g., 20 ft from street, 20 ft between structures, 10 ft from lot line for SF/duplex;…
- Front
- 20 ft
- Side
- 15 ft
- Rear
- 15 ft
- Max coverage
- 20% impervious
- Min lot
- 20,000 sf (~0.46 ac)
Principally residential village-center district (a ~100-ft band around Main Street); limited commercial by special exception (Sec 3.1.5). No frontage minimum stated (Table 1 row B = NA); minimum lot depth 100 ft if two-family residential.
- Front
- 40 ft
- Side
- 20 ft
- Rear
- 20 ft
- Max coverage
- 20% impervious
- Min lot
- 87,120 sf (~2.00 ac)
OVERLAY district covering government-controlled public lands (Great Island Common, Fort Stark, U.S. Table 1 still assigns it dimensional standards: min lot 87,120 sf, frontage 100 ft, minimum lot depth 150 ft, setbacks front 40 / side 20 / rear 20, max 1 unit per building,…
Form-based and downtown districts may use build-to lines instead of fixed setbacks (shown as “—”); verify your parcel's district before you build.
New Castle 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 New Castle (Rockingham County). A detached ADU can be up to 950 sf.
In New Castle's R-1 zone, ADUs follow the single-family setbacks: 20 ft front, 15 ft side, and 15 ft rear. New Castle has 6 zoning districts in total — see the full table below. Confirm your exact zone with the town.