Using SLC to bring existing floor height to the height of extension floor.

[Dumbo]

Probably a good idea to get flor level first but not built right up ufh so ufh boards dont rock . Fit ufh then bring up to level of rest of house with levelling comp.

 

[AP tiler]

They are pro fix panels Ultra Low Profile Underfloor Heating Panels - Floor Insulation Panels - http://www.profixpanel.co.uk/profix-panel.php.
supplied and fitted by Underfloor Heating from Underfloor Heating Systems Ltd - https://www.underfloorheatingsystems.co.uk.
They have advised the builder i'm working for to self level directly ontop of the panels/mats. They've gone ontop of the existing part of the house floor. in the extension part of the house the UFH is just a standard wet system under the screed.