Many areas in the city are designated as design districts. If the land use zoning of the property that you are considering has a "d" overlay (e.g. is zoned EXd or CSd), that means that any exterior changes to the building will need to go through the city's Design Review process. Design review may evaluate the architectural style, the building materials, the color, etc. to make sure that they meet the guidelines for that particular design district.
In some cases, objective standards in the zoning code may be used instead of design review. Community Design Standards may be used for some projects depending on the square footage of the exterior affected and what plan district the work is in. There are several types of design review and even the quickest can be expected to take several months. It may be possible to break out the exterior work from the remainder of the project in order to get started with the interior work sooner. Speak to a planner in the DSC, or call (503) 823-7526 for more information on design review and how it might apply to your exterior alterations.