Mine was a single storey extension and there were issues with the roof and footings. They were mutually sorted in the end but if rejected like ours the hoops multiply. With the footings we had to concrete in an RSJ.
Just had one done took around 15months from start to finish. Planning approval is the balls ache. We set 90k but with extras and unforeseens it came in around 110k. Still have odds and sods to do