Depends on the circumstances. My mum bought a house with an extension that had never been signed off. I think the seller had to arrange an indemnity policy? Didn't cost a lot anyway and it was no major hassle.What I was trying to convey was that in the case of building an extension to your home that it will have to comply with the Building Regs and in some cases the electrical installation is notifiable. Without the correct checks you then would not receive the completion certificate and that is a big problem if you try to sell the house.