Occupancy classification
Also called occupancy group, use group, change of occupancy
The code's categorization of a building by how it is used, which drives fire, egress, and structural requirements.
Occupancy classification is how the building code categorizes a building by use. It is one of the first determinations made about a project, because nearly every other code requirement follows from it.
The groups
The International Building Code uses letter groups: A for assembly, B for business, E for educational, F for factory, H for high hazard, I for institutional, M for mercantile, R for residential, S for storage, and U for utility and miscellaneous. Most have numbered subgroups, so R-3 for one and two family dwellings is a different set of requirements from R-2 for apartments.
What follows from it
Occupancy determines the number and arrangement of exits, fire resistance ratings, sprinkler requirements, allowable building area and height, plumbing fixture counts, accessibility obligations, and structural live loads. Two buildings identical in shape and construction can face entirely different requirements because they are classified differently.
Change of occupancy
Converting a building from one use to another is a change of occupancy, and it triggers a review against the requirements of the new group. This is where renovation projects become large: a retail space converted to a restaurant moves from M to A, and assembly occupancies carry heavier egress, sprinkler, and fixture requirements than the existing building was built for.
A change of occupancy requires a new certificate of occupancy. It is also a zoning question in parallel, since the new use has to be permitted in the parcel's district, and the two reviews are independent of each other.
Related terms
Rules vary by jurisdiction
This definition is general. How occupancy classification is handled in practice is set by the building department that permits your property. Find yours in our free directory.
Last reviewed September 4, 2026