Building sign-off can it be revoked?

We have had a extension built and signed off by the local authority but during the build a structural engineer had to do an extra calculation due to our builder ordering some steelwork that was a different spec to that in the plans. The steel work was fine and the engineer wrote and confirmed that this was the case, about 3 months on we get a bill from the engineer via the architect we passed it straight on to the builder, his mistake, his bill. While he has not paid it the engineer has written to say he is withdrawing his approval for the steel and revoking his approval claiming the building sign-off will be revoked. When I called him to say we would pay as the builder wasn't interested he told he wasn't interested in talking to me about it.

The architect has said he can't do this, anyone have any local authority experience of this nature, or have a view on where liability lies?

Much as it would be nice to have that power, it's not the case. If he had withdrawn the calculations from the LA prior to an approval being issued, then that would stop a full approval and completion certificate being issued- although a calculation could be provided by another SE (PII issues notwithstanding) to get the approval.

Not quite sure why, if you were willing to settle his account, he wouldn't take you money. Which makes me think that there's something more to this, of which you might not be aware.

Who instructed the SE to do the additional calculation? If it was you, then it falls to you and you need to then recover from the builder. If the builder did, but on your behalf, then the same applies. If the builder instructed him without your involvement and it was due solely to his mistake, then it is his bill to settle directly and is nothing to do with you.

If you've got your completion certificate, leave them to argue between themselves.