Will a lender give a mortgage for a property that will not officially be split until completion?

Will a lender give a mortgage for a property that will not officially be split until completion? - 1
Added (1). Scenario: seller has a mortgage on their 5 bed property and wants to split into flats. The lender will not allow them to do the split. Their mortgage value is only 166k.

We want to get a mortgage to buy 1 of the 2 flats for 200k. This allows them to repay their mortgage and keep the other flat.

Is it possible to get a mortgage on a property that has not yet been officially split, even if the work has been done? Can the split be done on completion once the sellers mortgage has been repaid?

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