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Bigger issue is that without a completion certificate, selling may be very difficult.
Sure but no specific time limit there and down to the risk appetite of the buyers, what their plans are etc. Some buyers want an old house with all the paperwork and assurances of a new build and are crazy IMHO.
My parents have been in the same house for 50 years and have done loads of things over the years. I expect I'll sell it after they die. I don't think the odds of me finding the building regulations sign off for various things that needed it 30+ years ago are high. I don't think it will have any impact on selling the house. Maybe I'm wrong, I'll report back one day.
Building regulation enforcement has to be within 24 months of completion according to https://www.planningportal.co.uk/info/200128/building_control/38/building_regulations/3