Legal Question in Real Estate Law in California

That's going to depend on the source of the leak. Condos are divided between owners' areas and common areas. Usually inside the walls are common areas belonging to the HOA, but not always every wall. Normally fixtures and pipes outside the walls are each unit owner's responsibility. Without knowing exactly where the leak is and what the condo documents say about who owns that part of the structure, there is no way to tell who is responsible. In an extreme case, however, you could probably sue the neighbor and get a court order to enter their unit to find the leak and stop it.

Mr. McCormick is the expert in this area, but I would just add that if you go to the extent of suing the HOA, why not also sue the neighbor in the same lawsuit? That would force them to argue to the judge as to which of them is not liable.