@Porterface84
A couple of pointers for you.
Remember, don't let your heart rule your head. Best advice I was given about property transactions, as a lay person not an investor, is to always remember this is the biggest asset and biggest investment you're likely to ever make and so therefore, you must be totally on top of the commitment and fully clued up on everything about it. What you need to know, is if I had to liquidate this investment in a heartbeat, could I do that?
If this property isn't compliant, then NO, you couldn't do that. The current estate agent and current vendor will tell you anything in order to make any of their problems shift to being your problems.
You really need to familiarise yourself with what permitted development means to this council, what planning permission is and crucially, all about building regulations approval.
I'm not up to date, but it used to be that you could apply for planning permission on land you don't even own, it's a theoretical thing. I was granted planning permission on an application with a set of drawings that my builder said just wasn't physically possible to carry through. Long story to do with roof lines.
But, without Building Regulations approval and sign off, any development is worthless. This is what you need to focus on most IMO. Often it costs more to renovate than to start from scratch.
Someone more up to date will come along and correct me where necessary, but I believe you should put your attention on to the current BR which will cover fire safety and so forth, as it's very clear that the development isn't compliant. It may have been a permitted development, but essentially it was never signed off as safe and legal and therefore, by definition, saleable.
I'm saying, those changes can cost an awful lot more than you'd anticipate and could be a super massive headache for you.
It's clear you love and want this house, but please be super careful.