During a section 32 inspection of a property by your conveyancer, they might see changes to the property via photos that haven’t had planning permission approval or event from the plans
When you purchase this property you are liable for this issue with a solution of either reverting the changes back or getting planning permission.
The next issue is if the council knows about this and this starts a timing of when the inspection is going to happen. The countdown is usually 21 days after the property has been inspected to start work. There is a possibility of other buyers contacting the council to try and drop the price or neighbors, or prospective buyers that didn’t win.
You have to get the council to inspect the property for you to give a list of issues to make the planning compliant. With this you need to go to a Building Surveyor who will cost out changes and planning.
I recommend you buffer in the cost of all the above into your property offer!