We've seen a lovely house with a kitchen extension out the back and a loft conversion. These were already in place when the current owners moved in and they've been there 12 years. We haven't asked yet but I suspect, in the way of these things, that they won't have, eg. building regs certificates for the extensions. The extensions appear sound, I checked carefully for any cracks, signs of damp etc around the windows and where the walls/ceilings meet. The house was nicely kept and in good condition, but not very recently decorated, so I have no concerns that there has been "papering over the cracks".
Would you absolutely positively demand certification for these kinds of works as a condition of making an offer? Again, we haven't asked yet. But I have asked in the past and the enquiry hasn't had a great success rate.
Please say no this is the first house we've both really liked!