I’ve read the comments and agree to some of them.
First of all, do you really like the house and are you happy with the price.?
Building regs have been around for ages but if you know anything about houses, the regs are not worth the paper they are written on. A classic example of this is new builds. They are mass produced very quickly but often to very poor standards but still pass regs. It doesn’t mean the structure of the building is any better and in most cases is often worse than house built 20 years ago.
It is very common to have house extensions with no building regs certificate. This never stops a purchase from going through and the indemnity insurance policy has been around for ages which proves it’s accepted practice. If it was so bad to not have a certificate, they wouldn’t have catered for an insurance policy to cover it.
As I said earlier, our house has several extensions with a missing reg certificate. However, I knew the value of the house and checked it over before buying and we have had no issues since.
A simple way to check if the electrics have been done to a reasonable standard is to view the fuse board. If there are separate RCDs for the extension then it would be safe to assume that the works were done properly. Obviously it’s better to have a certificate than not have one but it didn’t stop us buying our house and if you don’t buy the house you want, someone else will come and purchase it.
Might be worth asking the sellers if they took any pictures of the building works to show the different stages. When we did our extension we took lots of pictures with measurements so I can show to the next buyers the quality of the build work.
If you are going to be worrying every night that the house might fall apart or a fire might start then find another house and let someone else buy this one.
Finally, what kind of house are you buying? detached, semi? How much of the original house is still the same and what proportion of it is the new extension (missing the reg certificate)?
Do you know when the house was originally built?
All of this information will help you make an informed decision. Some people won’t take any risks in life, some will, it all depends on how big a risk it is and if you have any other houses lined up.