Concrete slab floor - getting it level in a single pour

[IanNE]

Intresting post thank lads..

 

[noseall]

Because an extension usually has a knock-through.

You mean a door?
I think you're saying it would be difficult to match the two floors at the same level.

I can see no reason why not.

It is not so much matching levels as dealing with the junction once the masonry has been removed. This is a vulnerable junction and will crack if not done properly.

It makes more sense to have a screed because the screed material as well as the insulative material can bridge the gap where masonry once stood and be seamlessly poly-floated into the existing floor screed.

Doing it your way means you have the existing floor then the new floor and a trough the thickness of the cavity wall in between. It gives no wriggle room if the existing floor runs out neither does it bond in one floor to the other at a vulnerable location.