Strafford County · New Hampshire
ADU rules in Somersworth, NH
Somersworth is a Strafford County city on the Salmon Falls River.
Somersworth is in our service area. This page covers what the town ordinance says about ADUs.
Town ADU rules
What’s allowed on your lot in Somersworth.
Somersworth replaced its ADU section in June 2025. An ADU can have up to 800 sq ft of habitable space and two bedrooms.
- Size limit
- Max 800 sq ft habitable space and 1,200 sq ft gross; max 2 bedrooms
- Where it can go
- Inside, added onto, or detached from a new or existing single-family house
- Owner on site
- Owner must occupy one of the two units
- Parking
- 1 off-street space for the ADU, on top of the house's parking
- Approval
- Permitted under §19.24 in districts that allow single-family homes; no special exception or CUP stated
- Water setback
- 50 ft no-build vegetated buffer from streams, wetlands and vernal pools inside a 100 ft riparian/wetland buffer district
Allowed
- ApprovalADUs are permitted on any property with an owner-occupied single-family dwelling that meets §19.24 criteria. The section cites RSA 674:71-73 and RSA 674:21. ADU is not a separate line in the Table of Uses.§19.24 (preamble)
- How manyOne ADU per property, in districts that allow single-family dwellings.§19.24.a
- Attached or detachedADU may be inside, an addition to, or detached from an existing or proposed single-family detached dwelling. Exterior alterations are allowed to fit the ADU.§19.24.b-c
- DesignAttached or internal ADUs: roof pitch, windows and eaves must match the house; must not look like a duplex or multifamily; similar exterior materials and complementary color; no fire escapes or exterior stairs facing a street; a separate entrance visible from the street is allowed if it does not look like a separate unit's entrance.§19.24.d.1.i-iii
- DesignAttached or internal ADUs must have an interior door between the house and the ADU; it does not have to stay unlocked.§19.24.d.1.iv
- DesignNew detached ADUs must be designed as an accessory structure with aesthetic continuity with the house. A converted accessory structure may keep its existing facade.§19.24.d.2.i
- LotA detached ADU may not be closer to the front line than the house, except: if the house is set back 100 ft or more, an ADU-only structure may be as close as 80 ft; an accessory structure built before June 16, 2025 may be converted; or the ADU is inside a multipurpose accessory structure.§19.24.d.2.ii(1)-(3)
- SizeMax 800 sq ft of habitable space (2018 IRC definition) and max 1,200 gross sq ft overall. No minimum stated.§19.24.e.1.i
- SizeNo more than two bedrooms.§19.24.e.1.ii
- LotADUs follow Table 5.A.1 setbacks, unless a legally nonconforming accessory structure is being converted.§19.24.f
- Owner on siteOwner must occupy one of the two units.§19.24.g
- ParkingOne approved off-street space for the ADU in addition to the house's parking.§19.24.h
- OtherNo condo or separate ownership; restrictive covenant to the City recorded at the Strafford County Registry of Deeds before the Certificate of Occupancy.§19.24.i
- SepticWithout municipal sewer, septic must meet NHDES requirements for combined demand. If no adequate state-approved septic plan is on file, a new one is required under RSA 485-A:38.§19.24.j
- OtherManufactured housing and townhouses may not be used as or converted to ADUs.§19.24.k
- ShorelandRiparian and Wetland Buffer District (authority RSA 483-B:8): buffer extends 100 ft from all streams, wetlands and vernal pools, widened for steep slopes over 20%. 0-50 ft is a no-build vegetated buffer; 50-100 ft is a natural woodland buffer where at least 50% of the area outside impervious surfaces stays undisturbed. Table 5.A.1 note 10 also says primary structures are set back at least 50 ft from the ordinary high water mark of rivers or mean high water of natural fresh water bodies (the note's district reference is not legible in the extracted table). State RSA 483-B applies separately.§19.13.6.A-D; Table 5.A.1 note 10
- State lawThe town text still requires an interior door and ADU-only design rules. State law lets the owner choose how an attached ADU is entered and limits design rules to those the house must also meet.RSA 674:72, III and VII
Not allowed
- Not allowedMore than one ADU per property.§19.24.a
- Not allowedADU over 2 bedrooms.§19.24.e.1.ii
- Not allowedCondo or separate ownership of the ADU.§19.24.i
- Not allowedManufactured housing or townhouses used as or converted to ADUs.§19.24.k
- Not allowedAttached ADU designed to look like a duplex or multifamily; exterior stairs or fire escapes facing a street.§19.24.d.1.i, iii
Read 2026-11-15 from City of Somersworth, New Hampshire, Chapter 19 - Zoning Ordinance (Cover: 'Last Updated June 15, 2026'; ADU section replaced by Ord. 19-25, amended 6/16/2025 (section footer says '6/15/2025 Section Revised in its entirety')). Anything not listed here, we check with the Somersworth land-use office for your lot.
Setbacks
Front, side and rear in Somersworth.
NH RSA 674:71-73 (HB 577) - one attached or detached ADU by right on a single-family lot
Somersworth's dimensional table, as printed in the ordinance.
| District | Front | Side | Rear | Lot / cover / height |
|---|---|---|---|---|
| AprimaryAgriculturalTable 5.A.1; Table 5.A.2; Table 4.A.1 (one-unit dwelling permitted). A frontage 150 ft (min lot depth 175 ft, note 1). | 35 ft (note 9) | 20 ft (note 9) | 50 ft (note 9) | 43,560 sq ft (net of steep slopes, floodplain, wetlands, roads, easements - note 13) · N/A (no max) · Height 50 ft |
| R-1Residential Single FamilyTable 5.A.1; Table 5.A.2. R-1 frontage 115 / 135 / 150 ft. | 25 / 25 / 35 ft (note 2: may align with block average, never under 10 ft or more than 25 ft required) | 10 / 15 / 20 ft | 30 / 30 / 50 ft | w/water & sewer 15,000 sq ft; w/water w/o sewer 25,000; w/o water & sewer 43,560 · N/A (no max) · Height 35 ft |
| R-1AResidential/Single Family/ATable 5.A.1; Table 5.A.2. R-1A frontage 100 ft. | 15 ft | 15 ft | 15 ft | 10,000 sq ft · N/A (no max) · Height 35 ft |
| R-2Residential/DuplexTable 5.A.1 and note 5; Table 5.A.2. R-2 frontage 150 / 175 / 175 ft. | 25 / 25 / 35 ft | 10 / 15 / 20 ft | 30 / 30 / 50 ft | Table shows w/water & sewer 20,000; w/water w/o sewer 27,250; w/o 43,560. Note 5: one dwelling unit is developed under R-1 regulations · N/A (no max) · Height 35 ft |
| R-2AResidential/Duplex/ATable 5.A.1 and note 6; Table 5.A.2. R-2A frontage 100 ft. | 15 ft | 15 ft | 15 ft | 10,000 sq ft for a one-unit dwelling (note 6) · N/A (no max) · Height 35 ft |
| R-3Residential/Multi-FamilyTable 5.A.1 and note 5; Table 5.A.2. R-3 frontage 150 ft. | 30 ft | 30 ft | 30 ft | Table row 26,250 sq ft (w/water & sewer); note 5: one dwelling unit follows R-1 regulations · N/A (no max) · Height 35 ft |
| MHManufactured HousingTable 5.A.1; Table 5.A.2. MH frontage printed '115 135' for the first row; 150; 80 ft (as printed). | 25 / 25 / 35 ft | 10 / 15 / 20 ft | 30 / 30 / 50 ft | w/water & sewer 15,000; w/water w/o sewer 25,000; w/o 43,560 sq ft |
| R/BResidential/BusinessTable 5.A.1; Table 5.A.2. R/B frontage 50 ft. | 10 ft | 10 ft | 15 ft | Residential uses 6,000 sq ft · Not given for residential uses (40% building coverage listed for commercial/office) · Height Not given for residential uses (35 ft for commercial/office) |
| R/CResidential/CommercialTable 5.A.1 note 5. R/C frontage per R-1. | per R-1 | per R-1 | per R-1 | Residential uses: note 5, one dwelling unit follows R-1 regulations · per R-1 (N/A) · Height per R-1 (35 ft) |
| R/OResidential/OfficeTable 5.A.1 note 5. R/O frontage per R-1. | per R-1 | per R-1 | per R-1 | Residential uses: note 5, one dwelling unit follows R-1 regulations · per R-1 (N/A) · Height per R-1 (35 ft) |
| BBusinessTable 5.A.1; Table 5.A.2; Table 4.A.1 note 2. B frontage N/A. | N/A | N/A | N/A | - |
| HMDHistoric Moderate DensityTable 5.A.1. HMD frontage 70 ft. | 15 ft | 15 ft | 15 ft | 7,000 sq ft (note 12: min lot area per unit 3,500 sq ft) · Not listed in Table 5.A.2 · Height Not listed in Table 5.A.2 |
Verified 2026-11-15 against City of Somersworth, New Hampshire, Chapter 19 - Zoning Ordinance. Towns amend ordinances, so confirm with the land-use office before you build. Official source
FAQ
ADUs in Somersworth.
- How big can an ADU be in Somersworth?
- Max 800 sq ft habitable space and 1,200 sq ft gross; max 2 bedrooms, per §19.24.e.1.i as read 2026-11-15.
- Does the owner have to live on the lot in Somersworth?
- Owner must occupy one of the two units (§19.24.g).
- Can an ADU in Somersworth be detached?
- Inside, added onto, or detached from a new or existing single-family house, under §19.24.b-c.
Planning an ADU in Somersworth?
Call (978) 987-0089 and we will read the ordinance against your lot. See the ADU models