Legal Question in Real Estate Law in California

This will be a difficult one. The fact that the remodel -- and presumably the fact that it was done without permits -- was disclosed, so you don't have the failure to disclose argument, which is powerful. Your best case may revolve around the nature of the "structural issues" you mention, and whether they were known and should have been disclosed, or whether, possibly, you may have a viable cause of action against a contractor. Without a lot more detailed and technical specifics, I can't say with any certainty, but I do believe it would be worth your while to do two things: have some (maybe two or even three) competent and licensed contractors give you estimates for the required corrective work, and then discuss their findings and all the other pertinent facts with one or more local lawyers who (a) have construction-defect litigation experience and (b) are willing to give you a free initial consultation.

I'm afraid you are completely out of luck on the disclosure issue, unless you have reason to believe and you expect to be able to obtain evidence that the seller knew that the remodel not only was done without permits, but was defective. A seller is only obligated to disclose what they know. After that it is up to the buyer to investigate, or take their chances. They are also not obligated to explain to you the consequences of un-permitted work. Once they disclosed the lack of permits, it was on you to figure out that that meant the city would have no record of the work and might require it to be torn out in whole or part.

As Mr. Whipple says, your only real possible path to compensation is if you can get a reputable contractor to say (and agree to testify in court) that the work was defective when done and that the defects are the result of the contractor's negligence. If you can get that, the next question is whether the contractor has enough money or insurance to answer for their negligence. If so, then you might have a path to recovery.