My name is ***** ***** I am a NSW Solicitor. Thank you for your question, and will do my best to assist you with your question. Please understand this is not legal advise Please understand this is not legal advise but a guide to assist you.
I am sorry to hear about this but the saying "Buyer Beware" apples in these cases.
In any contract there are clauses that say you have made your own enquiries before you entered into the contract and they included building and pest reports and also checking with council to make sure that they have approved all structures.
If the contract is misleading and gives you the impression that it is approved then you can sue on that. But generally the answer is no you have no legal recourse.
If the extension is less than 7 years old and was undertaken by a licenced builder then you may be able to sue them under the Warranty that they have to provide.
I hope this makes sense and is of assistance. If there is nothing further
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