Ground floor flat extension - anyone done this (or an expert)?

[LittlePickleHead]

We are looking to get our ground floor flat extended. One of the points another freeholder has brought up is the insurance provision.
Has anyone extended a flat before and can tell me how it affected the building insurance? How did you apportion the increase in premiums (I am assuming we would have to adjust our service charge to pay the extra each month?)

I am going to go back to them and say we will use a builder with public liability insurance, arrange separate building work insurance to cover us during the build, and to inform the buildings insurance and deal with the increased premiums.

Does this all sound correct and reasonable?