A doggie day care may be classified as either a "retail sales and service" or a "kennel" under the zoning code. That allows some flexibility as to location in either commercial zones or in industrial zones. If you have a particular property that you are considering, call the general information line for zoning at 503 823-1456. The planner can tell you whether a doggie day care would be allowed at that location and whether it would be considered a "change of use".
A change of use, for purposes of the zoning code, can require bike parking, additional vehicle parking and/or changes to the parking, or paving and landscaping on the site. Exterior changes to a building, in many areas of the city, must go through a design review process to make sure that the proposed changes meet any special design guidelines for the area in which the project is located. The requirement for design review can be triggered by a very minor change, such as installing a replacement window that is not identical to the original.