have you spoken to a company about the build cost? I did a side return (nothing fancy, but nice quality) and it cost me ?60k without any of the internal stuff. Then I spent ?30k on a kitchen, ?6k on my floor etc etc. It can easily go above ?100k I would say but you need a company who can help you keep the fluff to a minimum. I used hometales for mine and they advised on cost before I went ahead and they were spot on. Maybe try contacting them or similar? www.hometales.co.uk