I have a property on Mar Menor.

In Spain you must get a licence approved for any building work and pay the correct fee. If you have not done so then yes, they can make you restore the building to "as before" at your cost, plus a possible fine. If the builder has done the work without a licence that is down to you.

If you were Spanish this would be very unlikely to have happened.

Many thanks for replying. I had the work done I paid just over 2000 euros for an architect. I received the license at the end of May. And in June I was told that it was illegal. As far as I was concerned I did everything above board. There are many houses on the resort that are illegal, why have I  been picked out.

Sounds like you have grounds for an appeal...

Thank you.

Take a spanish speaker to the town hall and find out what the problem is...