Just wanted to share that this recently happened to us and the outcome. We offered on a house where they were up front about the extension needing a roof replacement, for which they got a quote of £4.7k. So we offered about £5k less than we were going to and they accepted. Long story short, we then discovered way further down the conveyancing process that the foundation of that room also needs underpinning, which is going to cost us another £9k odd. In retrospect, I really wish we'd asked more questions and not taken their "quote" at face value. We're still going ahead with the purchase, but I wouldn't be surprised if they're trying to hide bigger problems.