Thanks for your question.
You will not have a claim in negligence against then, it's a contractual property matter.
Your solicitor who acted for you should be your primary point of contact on this. You need to check what the Seller's said in written reply to the solicitor's pre-contract enquiries and see if there is any provable misrepresentation or untruth in their replies. If there is you may have a claim against them.
Utimately the solicitor should have explained to you whether or not the property had building regulation approval/completion certificate for the work and the consequence of taking out an indemnity policy , I would have expected him to.
The indemnity policy covers you (though do check) against the cost of the loss you suffer as a result of the local authority taking enforcement action against you. This would include the cost of complying with a notice served on you by the building control department, but until the local authority takes enforcement action you cannot use it to pay the costs of remedial works that you carry out privately.
It will also be a condition of cover in the policy that you should not inform the local authority of the problem you are having. So, unless the Seller has misrepresented/ lied on the issue OR you were not explained the consequence of taking the policy by your solicitor then I'm afraid that you're on your own.
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