Legal Question in Real Estate Law in California

Since you are talking about owners of apartments, I assume you are talking about a condominium. In that case, the owner of the source of the leak is liable to the downstairs owner. Whether the owner of the source of the leak is the owner of the upstairs unit, or the building homeowners' association depends on the location of the source of the leak and the terms of the condo deeds and CC&Rs;. So, just by way of example, if the owner of the upstairs unit had a leaky dishwasher, that would in almost all cases by the responsibility of the unit owner. However, if a soldered pipe joint in the wall of the upstairs unit failed causing the leak, then the HOA in pretty much all cases would be responsible.