ADU rules in Dover

"Accessory dwelling unit" is listed as a use permitted by right in every Dover zoning district table that permits single-family dwellings: R-40, R-20, R-12, RM-SU, RM-U, Heritage Residential, plus the Hospital, Little Bay Waterfront and Office districts. Section 170-24C expressly contemplates a detached unit, providing that if the ADU is located in a detached accessory structure it may be on either floor, and § 170-24B allows exterior alterations, enlargements or extensions of the detached accessory structure to accommodate it. Dover's definition in § 170-6B describes the detached option as a unit "constructed within a detached structure that is accessory and subordinate to a single-family dwelling," wording that reads most naturally as a conversion or expansion of an accessory building. RSA 674:73, rewritten by 2025 NH Laws chapter 197 (HB 577, effective July 1, 2025), independently requires every municipality to permit one detached ADU, so a purpose-built detached unit must be allowed regardless of how that definition is parsed.

The same by-right use listing covers attached and internal units, and § 170-6B defines an ADU first as "a secondary dwelling unit attached and subordinate to a single-family dwelling," which reaches basement conversions, interior conversions and additions. Section 170-24B permits exterior alterations, enlargements or extensions of the single-family dwelling to accommodate the unit, subject to one design rule: no change may make the house look like a duplex or other multifamily structure, and accessways built for the ADU must be placed to the side or rear of the building whenever possible. Only one ADU per property is permitted, and none at all on a lot that already contains more than one dwelling unit (§ 170-24A).

Section 170-24C sets the range directly: no less than 300 square feet and no greater than 950 square feet. The 950 square foot ceiling matches the default cap in RSA 674:72, VII, which also forbids a municipality from restricting an ADU to less than 750 square feet. A unit in a detached accessory structure may occupy either floor. The structure itself follows the district's outbuilding and accessory-use dimensional standards from the district table (for example 35 feet maximum height with 10 foot side and rear setbacks in R-40). Note that Dover's own ADU web page states a 350 square foot minimum; the codified ordinance says 300 and controls. A zoning amendment package posted July 28, 2026 for Planning Board review would add a measurement rule (exterior dimensions of the unit itself, excluding unconditioned space such as stairwells and utility rooms, with bathrooms, laundry rooms and kitchens counted), but it has not been adopted.

Section 170-24F states flatly that the property owner must occupy one of the two dwelling units; the ordinance does not say which one, so the owner may live in either the principal house or the ADU. Compliance is policed through § 170-24H, which requires a certificate of use from the Zoning Administrator verifying conformance with the § 170-24 standards, renewed by January 1 each year for as long as the ADU continues, at fees set in the city fee schedule (the city's ADU page describes this as an annual, no-cost permit of use). This requirement survived the 2025 statewide rewrite: RSA 674:72, VI still lets a municipality require owner occupancy of one of the units, require proof that one unit is the owner's principal residence, and adopt reasonable enforcement regulations, so long as it does not dictate which unit the owner occupies.

Section 170-24D requires a minimum of one dedicated off-street parking space for the ADU, on top of whatever the principal dwelling needs. That is the maximum a New Hampshire municipality may demand: RSA 674:72, IV allows up to one additional space per ADU where the municipality already requires parking for the principal dwelling. The statute also lets the owner satisfy the requirement either on-site or at a legally dedicated off-site location, at the property owner's discretion; Dover's text says only "dedicated off-street parking space" and does not repeat the off-site option, but the statutory choice is available.

An ADU is a use permitted as a matter of right in the districts that list it (§ 170-11A), so there is no Zoning Board of Adjustment special exception and no Planning Board conditional use permit. The administrative path is a building permit plus a certificate of use issued by the Zoning Administrator confirming that the standards in § 170-24 are met (§ 170-24H), renewed annually. Before the certificate of occupancy issues, the owners must also execute and record a restrictive covenant in favor of the City at the Strafford County Registry of Deeds barring condominium or other separate ownership, with a copy filed with the Planning and Community Development Department and the City Assessor (§ 170-24E). RSA 674:72, I likewise requires that one ADU be allowed as a matter of right.

Nothing in § 170-24 prohibits renting the ADU, and there is no familial-relationship or tenant-screening condition, but the owner-occupancy rule in § 170-24F means only one of the two units can be tenant-occupied at a time. Section 170-24E bars converting the dwelling, detached accessory structure and lot to a condominium or any other form of legal ownership distinct from the single-family dwelling, and enforces that with a recorded restrictive covenant running in favor of the City; RSA 674:72, I likewise prohibits separate condominium conveyance of an ADU unless the municipality allows it, and Dover does not. A full-text search of the whole Code of the City of Dover on eCode360 returns no short-term rental, transient rental or rental licensing ordinance; the closest analogue is the separately listed "bed-and-breakfast" use (a residential structure of nine or fewer bedrooms rented short-term, with common eating and living areas and 24-hour on-site management), which is its own use permitted only where a district table lists it. The ADU certificate of use must be renewed each January 1.