UK Property Law
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Thanks for your question.
Yes, you will have to mention it when you come to complete the Property Information Form because it asks about works. If you do not mention it and there is a problem subsequent to completion then you could get sued by the purchaser.
On viewings you are not under a duty to volunteer the information, but if you are asked directly then you should be honest and answer truthfully.
If the sound-proofing was completed with a guarantee then it will almost certainly not be a problem assuming the guarantee covers successors in title.
If there is no guarantee, then it depends on whether the purchaser considers it a problem. Some would, some wouldn't. In the event that they do then their conveyancing will ask you to pay for an indemnity policy to cover the cost of any remedial works which need completing in the future. Such a policy would probably cost around £100.00 approx.
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