The short answer: A change of occupancy happens when a space is put to a use that falls in a different occupancy classification (or a different occupancy group) than before — for example, retail (Mercantile) converted to a restaurant (Assembly), or a warehouse (Storage) to offices (Business). Because the code's requirements scale with occupancy, a change can trigger upgrades to means of egress, accessibility, fire protection/separation, structural loads, and energy — the new use must meet the requirements for its new classification, as governed by the IBC and the International Existing Building Code (IEBC). Not every TI is a change of occupancy, but when it is, the obligations jump significantly, and missing that is one of the costliest things to discover late. Whether (and how much) upgrade is required depends on the classifications involved and the adopted edition's existing-building provisions.
What counts as a change of occupancy
It's not just "new tenant" — it's a new classification. Mercantile → Assembly, Storage → Business, Business → Assembly (e.g., office to event space) are changes of occupancy. Staying within the same group (one office tenant to another) usually is not.
What a change can trigger
Because requirements scale with occupancy, the new use may have to meet, for its classification:
Egress — different occupant-load factors, exit counts, widths (Assembly is far more demanding than Storage).
Accessibility — bringing the space and path of travel up to current requirements.
Fire protection — different separation, rating, sprinkler, or alarm requirements.
Structural — higher live loads for the new use (e.g., assembly loads).
Energy — current energy-code provisions for the altered space.
The Existing Building Code
The IEBC governs how much upgrade an existing building must take on with a change of occupancy — it provides paths (prescriptive, work-area, performance) that determine the scope. The adopted edition and the classifications involved drive the answer.
Why it's high-stakes
A change of occupancy discovered after design (or after lease) can blow up scope and budget — assembly egress, structural loads, and fire separation are expensive. Flag it at the very start of a TI. See our TI checklist guide and means of egress guide.
Common review comments
New use treated as a like-for-like TI when it's actually a change of occupancy.
Egress/accessibility/structure not brought to the new classification's requirements.
Existing-building compliance path not identified.
Catch occupancy and egress issues before you submit