Maximum size of an ADU or DADU

When it comes to the maximum size of an ADU or DADU, Erik says there’s no one-size-fits-all answer. “It's different per specific area,” he explains. "The better way to kind of put it would be that depending on the area, the location, the city, it's all depending on what they're allowing it to be."

Erik notes that while some areas might limit units to 800 square feet, others could permit up to 1,200 or even a couple thousand square feet. The property size itself is another critical factor. A lot with only five feet of property on either side of the house won't accommodate a detached unit, but a large backyard—common in places like Seattle—offers more possibilities. In these situations, homeowners can often divide the space for a DADU.

Height restrictions also come into play, with many cities having regulations on multi-story units. On average, Erik says, most ADUs and DADUs fall between 500 and 1,200 square feet. He emphasizes that the best course of action for a homeowner is to check their local zoning regulations. A design and build firm, general contractor, or even the property owner can easily find this information by visiting the city and looking up the parcel's specific rules.