Kitchen extension floor problems advicen eeded

[Mercedes]

Our builders were ripping out our cabinets today and the floor covering. The floor's always had a bit of a slope but I put that down to concrete bit of floor as compared to floorboards. Some of the boards will need to be replaced which we half expected.

The problem is builders think the wall between kitchen and dining room is resting on the floorboards and in both rooms floor is slanting down under wall which makes it a real problem when trying to level floor

I'm terrified they will say we have to knock wall down and underpin or something. Has anyone any advice or experience? Is it hard or expensive to put right?

We inherited a very sloped floor in our kitchen, especially between rooms and this was an extension previous owners had put on. We solved the problem by having a few extra screeds done to help level it. It still slopes ever so slightly but you wouldn't notice if nobody mentioned it now... this was over a concrete floor. We then put Amtico down.

Turns out there's nothing supporting the floorboards as no joists or wall beneath floor. Help.