The City of Portsmouth passed a local ADU ordinance in 2017, which allows attached and detached ADUs by Conditional Use permit across many of its zoning districts. ADUs may be no larger than 750 square feet in most cases. Either the primary home or 12 Accessory Dwelling Units (ADUs) the ADU must be occupied by the property owner, and this must be verified annually. Since the law’s passage, 31 ADUs have been permitted and 25 have begun construction. The City’s Planning Board has considered ways to simplify permitting, as one estimate places a price tag of $20,000 to $30,000 in permitting costs alone. Several amendments have been proposed to the ADU law, with the stated aim to make ADU production easier.

• City of Portsmouth, “Land Use and Zoning Regulations” 
• Portsmouth Herald, “Accessory dwelling units in Portsmouth: 'Easiest' housing solution or is it too late?” (November 29, 2022)
• Portsmouth Herald, “As popularity of accessory dwelling units grows, Portsmouth looks to simplify permitting” (July 26, 2022)