My understanding is if you're looking for a mortgage, you would need to own the site. Your bank / mortgage broker would clarify for you quickly.
If you're building out of your own funds, you can technically go ahead, but you're leaving yourself open to legal headaches if anything goes wrong at any point. You don't want to plan for it, but small things can lead to big family arguments. It's much cleaner to have the ownership transferred to you. I'm not sure of the exact process, but you'd need a solicitor and would have to pay stamp duty on the transfer even if you're getting it for free.
It's common enough where for example someone is going to inherit a farm in the future, and wants to build a house now. My advice would be to have a quick chat with a local solicitor who'd be used to the practice.
Someone else may be more familiar (I'm from a farming background, but not the one taking over the farm).