ADU rules in Portsmouth

Portsmouth allows one accessory dwelling unit per single-family lot and lets the owner choose attached or detached. Table of Uses row 1.20, 'Accessory dwelling unit (Attached or Detached) up to 750 sq. ft. GLA', is marked P (permitted by right) in exactly the districts where row 1.10, single family dwelling, is permitted. A newly built detached unit follows the side and rear yard requirements of the district or Character District and must sit at least 5 feet from the principal house. Separately, Section 10.814.14 lets an accessory building that already existed be converted to a detached ADU notwithstanding the general conformity rule. This is the result of Ordinance #03-2026, adopted February 17, 2026 expressly to comply with state law: RSA 674:73, as rewritten by Laws of 2025, chapter 197 effective July 1, 2025, now says a municipality shall permit one detached accessory dwelling unit, where before it was purely optional.

An attached ADU (inside the house, a basement or attic conversion, or an addition sharing a common wall for at least 25 percent of the length of one side) is permitted by right in every district that permits single-family dwellings. Portsmouth goes further for attached units than for new detached construction: Section 10.814.132 expressly allows an attached ADU on an existing nonconforming lot and within an existing nonconforming building, provided no increased or new nonconformity is created. The unit may have its own entrance or share one with the main house, and the city may not dictate which (Section 10.814.30, tracking RSA 674:72, III). Only one ADU total is allowed per lot, so an attached unit and a detached unit cannot be combined.

Section 10.814.31 caps every ADU at 750 sq. ft. of gross living area, and the Table of Uses repeats the number in the use category itself. Gross living area counts finished, conditioned space measured to the interior perimeter and excludes decks, porches, garages and other unconditioned space, and for an attached unit it also excludes storage space, shared entries and anything not exclusive to the ADU. Section 10.814.23 separately limits an ADU to two bedrooms. Portsmouth is sitting exactly on the statutory floor: RSA 674:72, VII bars a municipality from restricting an ADU to less than 750 square feet, and sets 950 square feet as the default ceiling only where the municipality has not legislated its own number.

Either the main house or the ADU must be the owner's principal place of residence, and the owner has to produce documentation proving it to the city's satisfaction. If the property is held by an entity, one of the units must be the principal residence of a principal of that entity, such as a member or beneficiary. This is an ongoing obligation, not a one-time attestation: the Planning Department issues a certificate of use verifying owner occupancy and principal residency, and that certificate must be renewed every year with whatever documentation the department requires, with failure to comply treated as an ordinance violation. New Hampshire did not preempt this in the 2025 overhaul; RSA 674:72, VI still expressly permits a municipality to require owner occupancy of one of the units.

Section 10.814.26 requires one off-street parking space for the ADU on top of whatever the principal single-family dwelling needs. That is the most New Hampshire allows: RSA 674:72, IV lets a municipality that already imposes parking minimums on the principal dwelling require up to one additional space per ADU, and it gives the property owner the choice of providing the space on-site or at a legally dedicated off-site location, an option Section 10.814.26 does not spell out but cannot override. The general residential parking table, amended February 17, 2026 by Ordinance #04-2026 to comply with state code, scales minimums to unit size at 0.5 spaces for a unit of 500 sq ft or less and 1.0 space above that, with fractions rounded up.

ADUs are a by-right use. The Table of Uses key in Section 10.434.10 defines P as 'use is permitted in the district', and row 1.20 is P wherever single-family dwellings are P, with no special exception from the Zoning Board of Adjustment and no conditional use permit from the Planning Board. The city's application instructions describe a purely administrative path: talk to Planning staff, open an account in the online permitting portal, file a building permit application and pay the fee, with land use approvals flagged only if the specific project needs them. After the certificate of occupancy the Planning Department issues a certificate of use confirming compliance with Section 10.814, renewable annually. This changed recently: before Ordinance #03-2026 (adopted February 17, 2026) the Table of Uses split ADUs into administrative-approval and conditional-use-permit categories, and the city's 2025 ADU Handbook still describes that superseded process.

Nothing in Section 10.814 bars renting an ADU on a normal lease, and the point of the section is to add housing supply. Three limits shape it. The principal dwelling and the ADU may not be separated in ownership, including by condominium (Section 10.814.21), so the unit cannot be sold off. One of the two units must remain the owner's principal residence (Section 10.814.22), so an absentee landlord cannot rent both. And neither unit may be used for a business except a home occupation in the unit the owner lives in (Section 10.814.24). On short stays, Portsmouth's zoning has no short-term rental use category at all: Article 15 defines a dwelling unit to exclude 'such transient occupancies as hotels, motels, rooming or boarding houses', the transient lodging categories (boarding house, bed and breakfast, inn, hotel or motel) are separately listed and prohibited in the single-family residential districts, and Section 10.434.40 deems any use not specifically authorized in Article 4 prohibited in every district. New Hampshire has no statewide short-term rental licensing regime, and Portsmouth publishes no short-term rental permit or registration program.