Hillsborough County, NH Accessory Structures: ADU Rules (2026)

New Hampshire counties have very limited regulatory authority. They do not adopt zoning, building, or land-use ordinances - that power is delegated to cities and towns under RSA Title LXIV (Planning and Zoning). Therefore Hillsborough County itself has no ADU rules. The applicable framework is RSA 674:71-73, originally enacted by SB 146 (2016) and significantly strengthened effective July 1, 2025. RSA 674:71 defines an ADU as 'a residential living unit that is located on a lot containing a single-family dwelling that provides independent living facilities,' including provisions for sleeping, eating, cooking, and sanitation. RSA 674:72 requires every municipality to allow one ADU as a matter of right - either attached or detached - per single-family lot. Municipalities may not impose additional restrictions on lot size, setbacks, frontage, aesthetics, or design review beyond what applies to the principal single-family dwelling. The total living space of an ADU may not exceed 950 square feet unless the municipality authorizes more, and municipalities may not cap the size below 750 square feet. RSA 674:73 specifically requires every municipality to permit one detached ADU subject to the same standards. A separate electrical panel and electrical service to the ADU may not be denied. Where a municipality has no ADU ordinance, an ADU is deemed a permitted accessory use as a matter of right. Permits, inspections, and detailed standards are handled by each municipality's planning, zoning, and building departments (Manchester Planning Board, Nashua Building Department, etc.).