"A surprisingly large percentage of San Francisco’s rental housing inventory consists of illegal rental units, often called “in-law units” or “granny flats”. In recent years the San Francisco Board of Supervisors has made it easier to legalize some of these Unwarranted Dwelling Units (“UDUs”), while at the same time making it increasingly difficult for owners to remove UDUs from their property.

Owners of property with existing UDUs may legalize certain of these units as Legalized Existing Dwelling Units (“LEDUs”) through the City’s Dwelling Unit Legalization Program, subject to compliance with life-safety standards and other restrictions.

Several years ago, San Francisco initiated a trial program to legalize certain empty spaces adjacent or near to existing legal dwellings as Accessory Dwelling Units (“ADUs”). Originally available only in targeted neighborhoods, San Francisco’s trial Accessory Dwelling Unit Program has been expanded to all zoning districts that permit residential use.

An ordinance imposing a 10-year wait before nonconforming residential units may be enlarged or remodeled following an Ellis Act eviction was struck down by the courts in May of this year (Small Property Owners of SF Institute v. City and County of San Francisco, 22 Cal. App. 5th 77). This may eliminate similar restrictions on adding ADUs."

http://g3mh.com/wp-content/uploads/2018/06/2018-ADU-FAQs.pdf