Survey request after signing party wall agrement

[Kiaara]

We have started work on a side and rear extension. We already have a party wall agreement signed by our neighbours before commencing works.

However 3 months into the work they have reached out to us saying they want to appoint a surveyor to review the extension work and pass the cost on to us. Is that possible to request? They didn’t raise any of these conditions while signing the party wall agreement.

What do they want to survey? That the extension is being built to the designs?

No, if they want to do that, that's up to them.
Assuming you have all the relevant permissions in place and a party wall agreement already they have no reason to pass any costs on to you. Even if you don't have all the relevant permissions that's your issue not theirs! If they are concerned about anything have they raised it with you? They can raise any concerns with Building Control or the planners if they choose. But that's up to them if they aren't happy.