Buying part of flat from family member

steampowered

Perhaps your aunt could sell the flat to you for 300k; the bank would grant a mortgage over the entire flat; the sale contract would say your aunt gets half the equity if the flat is sold in future; and your aunt's right to get half of the equity on a sale can be secured by a second ranking charge?

The other alternative would be to buy 100% of the property from your aunt; pay her 300k up front using the mortgage; and agree pay her the rest over time under a separate loan agreement. This could either be monthly payments or the payments could be deferred until the property is sold.

Both of those options shouldn't cause you problems with the mortgage lender as they would simply be lending 300k against a 700k property. The mortgage lender is not interested in the terms of the sale between you and your aunt, so long as its gets security over the entire flat.

A competent solicitor should be able to document all that. A cheap conveyancing shop would have no chance. Whether its a good idea or not is another issue.