Before you invest in a new property, we recommend you thoroughly research the property. The Planning Department and the Department of Building Inspection keep records on San Francisco's parcels and properties. Using the Planning Department's San Francisco Property Information Map (propertymap.sfplanning.org) you can look up the Zoning District, Permit History, Case History, Historic Preservation information, and much more for each property in San Francisco.
Rules of Thumb
Generally what establishes legality of a use or a structure is a building permit. The Department of Building Inspections's Records Management division, 1650 Mission St., 4th floor, 558-6080, is the keepers of those records. They are also responsible for 3R reports (Report of Residential Record). The 3R report provides a summary of building permit history for the property and indicates what the Department of Building Inspection (DBI) considers to be the number of legal dwellings currently on a site.
The Assessor's Office also keeps records of land use. However, these are not always reliable. For example, the Assessor's Office records will recognize existing uses without verifying they are legal. So often the Assessor's Office may show a building containing more dwelling units than were created with proper building permits.
Similarly, an approved building permit is definitive as to the legal use on a site rather than the Assessor's Office listings. For example, if the Assessor's Office lists a property as "Industrial" and the last approved building permit for the space in question was for office use, then the use is legally office space.
However, in absence of other records, Assessor records may be helpful in indicating how a property has been used over time. Contact Records Management/3R for guidance when building permit history is ambiguous or lacking.
Please note, realtors will sometimes advertise properties containing "unwarranted" space or "unwarranted dwellings." These dwellings are often "in-law" units, usually a minor unit at the ground floor. The listing of space or units as "unwarranted" indicates that its legality is not verified. Only the building permit history, and the associated 3R report in the case of dwellings, is the evidence for legality. It is important to verify that dwellings are legal as often they cannot be legalized due to zoning density limitations. Further, fees and penalties may apply to legalize the space. Worse yet, if tenants occupy illegal units, additional costs may accrue to a new owner in any eviction process required to legalize the occupancy of the space.