According to this website, it looks like 871 14th St is zoned for 40ft in height and is zoned RTO (Residential Transit-Oriented Neighborhood). This basically means that it's a neighborhood consisting of single family and multi family dwellings that is very near to public transportation and less than 1/4 mile from a commercial area. Many homes in these zoning districts are 3+ units, but some are single family or 2 family homes. This particular property appears to be single family home with 2 stories and 1920 sq ft.
Of course, if you want more detail regarding what the city will allow you to build on this lot, or add on to this building, you'll want to contact the building and planning departments in SF.
If you'd like to discuss this property more, feel free to contact me!
Hope this helps,