Hello Sang, and thanks for your question.
Unfortunately, much as we'd all like to provide you with a legal answer, as Realtors, we're woefully ill equipped to render a legal opinion, and your question is a legal one. However, I think I can steer you towards a department that can answer your questions without charging you any money, and that is to encourage you to contact the Planning and Building Department of the City and the Assessor's office of the County of San Francisco.
You do not need to provide an address in your question, but it's best to learn directly from the professional in each of these departments how to obtain a permit for an already built item, and to determine how this might affect your property taxes. You need to invest some time with the County and City in obtaining an answer, but it will be time well spent (and without charge) to learn how purchasing this property may affect your ability to sell the home in the future, and to maintain certain aspects of the property.
Grace Morioka, SRES, e-Pro
Area Pro Realty
San Jose, CA