[toyg]

If you infringe building regulations, yes, you can be ordered to redress the situation at your expense. Typically that means demolishing illegal structures; in this case it was the opposite, which is pretty rare but can happen.

For the record, this is common throughout Europe, it's not a UK-specific thing.

> three years after they bought it, the law was changed

That's tough, but dura lex sed lex.