When should we start insurance?

We're about a week away from exchanging contracts on a new build property but it won't actually be ready until November/December this year. All the information i've been given says we should begin life insurance and home insurance immediately after exchange. Is this really necessary? Paarticularly in the case on building insurance at the moment there is no building as not even the foundations are laid yet. Does anyone have any advice as to when i should take these policies out?

Thanks