Am not sure I’d be able to do that, it’s attached to another house; planning permissions I imagine would be a nightmare; it’s likely they’ll balk at any reductions in price let alone a massive reduction; and iirc the terms of my mortgage are specifically for a house purchase not a house build.
Dream idea though, I was looking at land the other day thinking how cool it would be to be able to just buy some land and build a house on it. Potentially cheaper than buying, just a huge project to manage.