We are in the process of buying a bungalow that was built in 2004 with no completion certificate sought after the time of the build. There was planning permission in place but no sign off, so it wasn't checked to make sure that it complied with the plans submitted and to regulations.
For us, as it was the whole bungalow, it felt too risky to proceed with the purchase when, for all we know, it could have been built by cowboys who cut corners at every opportunity.
Our Solicitor asked our buyers to get the building signed off retrospectively by the council. So this is what they did.
The process was fairly quick to be honest. They came and did a proper check. There were things that were not to regulation that needed to be put right, which our buyers did. The council then came back to re-check and then issued a certificate of completion and signed it off. The whole process only took about 4 weeks from first check so it shouldn't set you back too long, unless the council find lots of things that need to be rectified in order for you to be issued the certificate.
You may find this is the most sensible thing for you to do. Yes, it may cost a little to bring certain things up to standard but you may well find you will need to do this for anybody wanting to purchase, not just these buyers.
Honestly, our Solicitor said we could walk away but we wanted the bungalow and were prepared to wait. Our sellers solicitor advised our sellers to get the build signed off retrospectively also. They said that any good Solicitor will want this doing before they would advise their client to purchase. So if you don't do it for these potential buyers, you may well just end up having to do it anyway.
Just for comparison, selling our own house we have had to jump through hoops with boundaries, easements and all sorts of nonsense that no-one gave a toss about when we bought our house 15 years ago. Things we have never had a moments issue with. Its VERY annoying that suddenly these things are now issues but its just the way things are now. Everyone seems to want everything to be perfect.