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The good news is that there are several different avenues that you can take in order to get the green light from the local council.
However, how laborious this process will be - and the costs involved - depend on when and how the work was carried out.
The best case scenario is that the work comes under something called permitted development rights, which would mean it never required planning permission in the first place.
On the other end of the scale, you may need to apply for planning permission and then have it refused. This would make you responsible for restoring the property back to how it was before the second storey was built.
A house we want to buy was extended without planning permission thisismoney.co.uk - get the latest breaking news, showbiz & celebrity photos, sport news & rumours, viral videos and top stories from thisismoney.co.uk Daily Mail and Mail on Sunday newspapers.