Legal Question in Construction Law in California

Your question doesn't give enough facts. Does the contract specify where notices are to be delivered? Is the owner or the occupant the person who signed the contract? What is the relationship of the occupant to the person who signed the contract? Did the contractor have an obvious better choice at to where and whom to serve? Overall, it may be wiser to file an answer to the complaint and raise improper service as an additional defense, but I'd at minimum recommend an immediate conference with a prospective defense attorney.

It depends on what "papers" were served. If it was a summons and complaint for a lawsuit, that would not be effective service regardless of the answers to Mr. Whipple's questions. If it were a mechanics lien notice or any one of a number of other "papers" that might be delivered in a construction dispute, then it might be. As Mr. Whipple says, you should talk to an attorney, especially if what was delivered was a summons and complaint, because that you are going to have to respond to eventually.