Advice on an unlevel house

FHB brought an old 1970s house, we did a building and pest which flagged the house as 35mm but said this wasn't "deemed excessive" so we stupidly didn't much of it. Flash forward to trying get new floors put in we've realised this is actually quite a bit out and that would need to structurally rectify it.

To give an idea the worst of it seems to be the hallway that's runs right through the kitchen and the bedroom on the wing. It's like dipped in the higher traffic areas.

I understand I need to get under the floor and try and diagnose to get an idea and get someone out. I'll be there next week to do that.

But as I'm kicking myself for not negotiating this with the seller, what costs am I potentially looking at to rectify this? I should clarify it's on stumps.