The building code sorts the ways that buildings are used into separate "occupancy classifications". Every building is given an occupancy classification when it is built, and each occupancy classification has different building code requirements that go along with it. The requirements reflect the type of hazard or uses in the building. A building will remain "grandfathered" even when the code requirements for any occupancy classification change over time, so long as the occupancy remains the same. A change of occupancy is when there is a change in any part of the building's use that would place it in a different occupancy classification.
Changing a house to include some commercial use is a "change of occupancy" from R3 to a mixed occupancy. A mix of office and residential would be classified as a B/R3, or retail and residential would make it an M/R3. When the occupancy of a building changes, that triggers some building code requirements as if it were new construction; aspects of the building that would have been grandfathered under the original permitted use need to be addressed in the light of current code.
It is not uncommon, in the case of a change of occupancy, to have one or more building code appeals. The process takes about a week and the appeal fee is typically $200. View more information on the appeal process.
The Life Safety reviewers are the city staff who work on building code issues. Their voicemail/info phone number is (503) 823-1456. Life Safety reviewers can also meet with you in the Development Services Center.