ADU Design in Portsmouth, NH

ADU Design in New Hampshire: what governs your project

New Hampshire's 2017 law (RSA 674:71 through 674:73) requires every municipality to allow at least one attached accessory dwelling unit by right wherever single-family homes are permitted. Municipalities may still regulate size, aesthetics, and separate entrances, but they cannot ban ADUs outright.

Energy code

Local rules typically cap the ADU at a fraction of the primary home's size, may require owner occupancy of one unit, and use the standard residential building-permit path. Sewer and water connections follow the local utility's normal process.

Permitting & plan check

Permitting is municipal, handled by planning boards and building departments in each city and town. There is no broad state building-permit layer, but NHDES wetlands and shoreland reviews run alongside local approvals for affected sites.

Good to know

• New Hampshire's Shoreland Water Quality Protection Act regulates development within 250 feet of public waters.
• Mount Washington has recorded some of the highest surface wind speeds ever observed on Earth.
• New Hampshire levies no general sales tax and no tax on wage income, which shapes commercial development economics.

Every discipline, one team