Mortgage start date is before seller can vacate the property

Hey guys,

So my partner and I are FTB and are getting close to exchanging contracts, everything has moved insanely quickly as there's no chain either side, (seller is buying a new build property).

Problem is the property is not yet built, the builder has estimated it will be ready for around August time, however, we've noticed the start date for our mortgage (which is now approved and offered), will be before this, (30th of June).

What happens in this situation?

We can't really give a concrete date to our mortgage lender because the seller hasn't got a concrete date as to when their house will be finished, so our mortgage advisor just went with 30th of June.

We also got sent a bunch of forms to sign from our solicitors a couple days ago, and on one of them it states that if no fixed completion date can be given, the onus will be on us to ensure we have a valid mortgage offer in place, and also that the searches will expire after 6 months so we may have to pay to have those done again.

Feel as if everything has moved so quickly that now everything revolves around when the sellers house is built, which we can't they can't even guarantee!