Insurance needed for annexe while main build continues

[Grendel]

Our builder has completed the annexe which we are now living in. He's carrying on with the main house which is half done.

We need building and contents insurance for the annexe but are having trouble. 

The builder says that as we are living here it's no longer covered by the site insurance.  But the insurers that we have contacted say they can only insure once BCO has given sign off. But BCO won't be able to do sign off until main house is also completed. 

This can't be a new problem.  Has anyone else solved it before??