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Under Floor Heating 
Anyone on here deal with wet underfloor heating? 
I`m looking at getting an extension built to the rear of the house, moving the back wall out 2-3m across the full width of 8 meters of kitchen dinner. I`d rather not dig up the existing concrete slab due to costs and don`t want to raise the floor level approx 100mm to accomodate insulation a screed. Hence I like the low profile systems so can keep the existing floor level. 
Are they any good and any recomendations

I assume you are intending to run the underfloor heating under the new floor(extension) and the existing floor. Obviously the new floor will be insulated to current specifications, if the existing floor has no insulation you are best excavating it and insulating correctly.

Certain parts of the country have different specs for insulation under building control. If I was doing it on y own house woul use 150mm king span/celotex to maximise efficiency

Yes I will be running the ufh in both new and old areas.
The existing house is only 12 years old so assuming there would be some insulation under the slab already?

There should be some insulation under there, I doubt very much it will be anything like the current specs. Could be 50mm polystyrene. Have you had a price to dig the floor up and continue the same floor throughout?
Underfloor heating with poor insulation is not as efficient an you would actually feel the difference between old and new if barefoot

Not had a price to dig it up yet, approx 21sqm would need removing. just thought it would be dearer than the difference in low profile systems to normal systems if that makes sense.
Hadn't thought about the new and old giving a different feel under foot but that does make sense and as it happens the low profile systems are coming in a little dearer than expected so maybe digging would be equal cost.

Not to mention the saving on fuel bills as more insulation means less heat loss and therefore more efficiency!

It's not actually too bad to dig up once you are started with a big breaker, we do atheist one floor a week, doing loads of garage conversions at the moment, concrete has to come up to insulate.

All the best