Do I need to "level" the subflooring before laying engineered wood flooring?

RoseLondon

It came as a bit of surprise today when I received the quote from the fitter showing the quoted price nearly doubled because they want to use some mix compound to "level" the subfloor. Our floor currently has half old concrete flooring and half newly screeded flooring. The screeding was quite well done and levelled. But there's gap between the two old and new floorings meet. And the old flooring obviously isn't in perfect condition.

I am surprised purely because until this point of time, I've never seen anywhere mentioned above tHis process. We are going for overfloat way of laying the engineered wooden floor and we will have underlay. We thought the underlay would sort of help even the not so perfect concrete subfloor. The fitter first suggested £18/m2 without mentioning of this extra process of levelling. Now the quote rose up to nearly £40/m2. (We supply our own wood planks and underlay)

Is this reasonable?