Laying flooring over differing subfloors

[taybert]

We’re knocking our kitchen and diner together to make one big room and I’m thinking about flooring. I’m probably going with some sort of LVT but the problem is that the current kitchen has a concrete floor and the dining room has a suspended wooden floor and the prep is usually different. Does anyone know if it’s possible to just lay plywood across the whole thing to make a uniform surface to lay the flooring on to?

I have this same issue but I'm planning to tile. Kitchen floor is concrete, dining room is suspended wooden and then theres the new concrete floor in the extension. When the walls came down between kitchen/dining and dining/extension we could finally see the proper floor levels. Which are not level :-(
The builders were planning on something called cement board on top of the floorboards to make it stable enough to tile on. Now it looks like there may also need to be some self leveling cement somewhere too. The floorboards weren't level, the kitchen has been raised up by about a centimeter at some point and although flat, is not level. Apparently it's a bit of a pig of a job now but can be done.

Yeah, that’s the other issue, the floors must be roughly level but rough is no good when it’s one floor throughout!

I extended our kitchen (suspended floor) into an old outbuilding and the builders laid a new concrete floor to that section.
Where the concrete floor met the suspended floor, the builders left a layer of celotex.
I decided to go with engineered wood flooring and every fitter that came round had a different view of how to solve the issue of two different subfloors.
In the end the guy we went with overboarded the wooden floor with ply and used self levelling compound to match the levels on the concrete area.
The new floor squeaks at the joint, but in reality, I'm not sure there was any alternative solution that would have been better Im afraid..