Floor levelling problem

What was an integral garage has at sometime in the past (before I owned the property) been incorporated into my lounge. I wish lay engineered floor boards over all of the lounge including what was the garage.
However, the garage part has a floor level of 1-1.5 cms higher than the lounge part (variation due to bad screed laying).
The screed layer is about 4 cms thick so if the screed is removed and relaid then it will only be 2 cms thick.
The lounge area is four times the "garage" area if that's relevant.
Any sugestions/advice very welcome.
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