What if the seller claims they genuinely didn't know about the defect?

This question typically represents the entire core dispute in a non-disclosure case, since a legal claim for non-disclosure fundamentally requires you to prove the seller actually possessed knowledge of the specific defect at the time of the sale, mere existence of a defect the seller genuinely didn't know about generally isn't actionable under this legal theory. Strong evidence of actual prior knowledge includes things like prior repair invoices for the same issue, permit records held on file with the local city or county government, a past insurance claim filed related to the same problem, an old inspection report from an earlier sale attempt that ultimately fell through, or even a neighbor who directly witnessed the original problem occur or get patched up.

Without at least one piece of this kind of documentary or witness evidence establishing the seller's actual prior knowledge, rather than simply the fact that the defect existed, your case becomes considerably harder to win, since you're essentially asking a court to infer knowledge from circumstances alone rather than pointing to concrete proof.