My parent's house built around 1980 .... has a living room, to which was added a small extension both parts have solid screed floors.
Living room originally had a 6' window .... this was removed and room extended full width out by around 3m.

Currently has a pretty awful laminate floating floor laid over it, there is slight but a definite €˜hump as you move from extension to main room.
At the point where the original external cavity wall was (now under the screed)

The is at 90 degrees to plank run.
Going to need to relay flooring ...will add a 3mm underlay to try and help even out .... Q is whether fitting a Vinyl plank flooring might be better for this than laminate ?

Don't want to consider replacing screed as it would mean removing skirtings, replastering walls etc.