Nassau County, FL Accessory Structures: ADU Rules (2026)

15(K) treats accessory dwellings differently from the ordinary conditional-use 'mother-in-law dwelling' the code also recognizes: an accessory dwelling is permitted by right in every residentially zoned district that allows single-family homes, whether built alongside a new house or added to an existing one, and only one is allowed per single-family parcel. Size is capped at 50 percent of the primary home's square footage, excluding garage space from that calculation, with an absolute ceiling of 800 square feet no matter how large the main house is.

Design standards require attached accessory dwellings to meet the underlying district's ordinary setbacks, while detached units need only a six-foot setback and cannot sit in a required front yard, though on parcels over three acres an accessory dwelling can go between the principal structure and the street outside that required front yard. Height tops out at 25 feet, and exterior entrances and staircases must stay at least six feet from an adjoining property line. An extra off-street parking space beyond what the single-family home already requires must be provided on-site, and new parking has to use existing curb cuts rather than adding new ones.

When a rental is involved, subsection (K)(10) requires the existing single-family home on the parcel to remain the owner's homestead-exempt primary residence, and (K)(9) blocks selling the accessory dwelling separately or splitting the parcel to isolate it. A recreational vehicle, mobile home, or storage shed can never substitute for an accessory dwelling under (K)(7).