Your builder is liable for his actions whilst he is at work, thats why he has public liability insurance! Just because you are the client and he is working on your property does not make you liable. If he injured or killed himself it would be his fault, so the same applies to loss and damage. If your neighbours property has been damaged then he needs to supply a claim form, from his insurers so that your neighbours can claim for the repairs.

Was the damage caused by your builder during the course of his employment building your extension? If it was, then it is up to your builder to rectify the damage and to reinstate your neighbours property to the same condition prior to the damage.

This is why we have Public Liability insurance just in case these events happen. If your builder has such insurance then your builder, or your neighbour, may make a claim to cover the cost of repairing the damage.

Your builder is responsible for any damage which they may have caused unless he got you to sign a disclaimer to say that he wont be held responsible for any damage caused

You are liable for any claims by your neighbour. If these claims have arisen as a result of your builder's negligence then you have a possible claim against him for your losses. He should have insurance to cover this. The insurer may choose to settle directly. If no negligence is involved then it is more complicated. In the first instance speak to your home insurers if this is the case. Good luck

if a party wall agreement was put in place prior to building works and builder signed and agreed the he should ,if not its a grey area ,builder only carrying out your instructions ,if it was not due to poor workmanship you the homeowner who should have a party wall agreement is liable

That would be your builder. Whom should have suitable Construction All Risks Insuramce.

Your builder should have liabillity insurance to cover such damaged,you should have checked this before you let him start work on your house.

The builder.If it was unavoidable,he should have told you at the outset.