If you want to build a 6 unit residential building in the Bay Area you first have to buy the land it will go on, and then build a building on it which will tie into and distribute utilities for electricity, water, sewage, and possibly gas.
The cost of finishing out each unit within goes on top of that foundation which already existing in this case, where the units were converted offices inside existing buildings already owned by Caltrain.
Maybe it should be easier to rezone and repurpose existing land and buildings into residential units. A lot of the regulations that hinder conversion of non-residential buildings into residential are there for good reason, for things like ventilation and fire escape. Regulations written in blood after previous tragedies.