The International Building Code provides several routes for designing buildings with more than one occupancy. Building owners who don’t want to separate occupancies with fire separations have to employ one of these. For buildings with a large primary occupancy and a small accessory occupancy, the answer may be section 508.2 – Accessory Occupancies.
An accessory occupancy can take up no more than 10 percent of the building area on the story where it is located and can’t exceed the Table 503 values. Note this is area per story and not as a percentage of the total building.
The area of the accessory occupancy must meet the requirements of that occupancy.
No separation is required between accessory occupancies that meet these criteria and the primary occupancy with a few exceptions:
Group H-2, H-3, H-4, and H-5 occupancies must be separated from other occupancies. Group I-1, R-1, R-2 and R-3 dwelling and sleeping units must be separated from each other and contiguous accessory occupancies. Incidental use areas must still be separated.