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 that detached accessory structure to accommodate it. Dover's § 170-6B definition covers both forms in a single sentence and 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 ch. 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. Since August 31, 2026 a unit completely contained within a preexisting detached structure counts as a detached unit under RSA 674:71, III (2026 NH Laws ch. 210), but nothing in Dover turns on that label: § 170-24 applies the same standards whether the unit is inside the house or inside a detached accessory structure.

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). Dover imposes no standard that differs between an attached and a detached unit, so the reclassification worked by 2026 NH Laws ch. 210 changed nothing here.

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 the Planning Board posted on July 28, 2026 would add a measurement rule (the area is the unit's exterior dimensions, excluding unconditioned space such as stairwells and utility rooms, with bathrooms, laundry rooms and kitchens counted); it is still only a proposal, set for Planning Board public hearings on September 8 and September 22, 2026.

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 and the 2026 amendments: 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, and that is the only parking the zoning ordinance counts for a house with an ADU: Article XII (§ 170-56, amended October 8, 2025) sets design and screening conditions but no numeric off-street minimum for the single-family dwelling itself. Two state rules cap what Dover may ask for. RSA 674:72, IV lets a municipality that requires parking for the principal dwelling require up to one additional space for each ADU, and it lets the owner satisfy that space either on-site or at a legally dedicated off-site location at the owner's discretion, a choice Dover's text does not repeat but cannot override. RSA 674:16, VII, in force since September 13, 2025, separately bars any New Hampshire municipality from requiring more than one residential parking space per unit, so no second space may be demanded for either dwelling. Dover does not require the space to be covered, which matters because 2026 NH Laws ch. 329 bars a municipality from requiring that the per-unit space be garaged effective September 13, 2026.

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, occupant-count or tenant-screening condition anywhere in the code, 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, vacation 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.