Accessory Dwelling Unit (ADU) Rules for Marlow, NH

ADU State Law

What can you build in New Hampshire?

• –New Hampshire ADU rules after HB 577 (signed July 15, 2025, eff. July 1, 2025 – Chapter 197).
• –One ADU, attached or detached, is allowed By Right on every single-family lot in any zoning district that allows single-family homes.
• –If the local ordinance is silent, the same By Right rule applies. You just need a building permit.
• –Default maximum is 950 sq ft of living space
• –Towns may set a lower maximum, but no lower than 750 sq ft,
• –Town may set a higher maximum
• –If a town specifies a size limit only for Internal or Attached ADUs and is silent on Detached units, we interpret that the state default maximum of 950 sq ft applies to Detached ADUs, since local limits apply only to the types they explicitly regulate.
• –Local zoning rules for ADUs can't be stricter than those for single-family homes, including lot size, frontage, and setbacks.
• –May require up to one additional parking space for each ADU.
• –Owner-occupancy: A town may require the owner to live on the property, but can't say which unit the owner must occupy. No requirement that ADU occupants be related to the main-house occupants.
• –Water & septic: The town can't demand a bigger system than the state DES rules require. Separate systems aren't mandatory, but you must upgrade if the existing one fails or lacks approval.
• –Conversions encouraged: Existing structures (e.g., garages, barns, etc.) may be converted even if they violate current setbacks or lot-coverage limits, and towns can't block a separate electrical panel/service for the ADU.
• –A town may bar ADUs on leased land or on lots with multiple attached single-family dwellings (e.g., townhouses).