Financing a conversion with a planning condition to the main farm house

NatalieCWalker

I’m after some advice please knowledgeable folks, this is the situation;

My mother owns a house with a barn on the same plot. My husband and I have been successful at gaining planning permission to convert the barn for us to live in, but only as an ancillary annexe to the main dwelling my mum lives in. We’ve sold our property to part fund it, but we’re about £80000 short of the projected build costs. How can we finance the remainder? The title deeds are currently all in my mothers name, we considered splitting the barn into our names to get a mortgage but with the planning conditions as an annexe I’m not sure it’s mortgagable? There’s 5 acres of land could we get a mortgage against that?
Would appreciate any suggestions of how we might progress the situation. Many thanks in advance.