What depth do i remove to replace a kitchen floor?

[Erdinger]

Have removed the old stone tiles, ready now to dig out the underlying material. Ultimately i will have underfloor heating installed over a concrete base, then tiled. Can anyone recommend to what depth i should go down... Thanks

[buildersmate]

Electric mat or flexible pipe with hot water running through it? - though to be honest you need a similar base for either.

2 critical things to remember:
1) Because the floor is going to get hotter than normal, you need a lot of insulation below the floor slab - otherwise you lose too much heat into the ground.
2) the thicker the floor-slab, the longer the heating takes to heat up (and cool down) - its a thermal mass thing.

1 is sorted by putting at least 80mm of Celotex below the final floor slab. But put more if you can afford it - see 100-120mm.
2 is sorted by making the final slab (above the Celotex) thinnish by using a liquid screed - down to say 45mm. Otherwise you could use concrete of say 65mm thick.
Below the Celotex you need about 100m of hardcore, then 60mm oversight concrete, then damp proof membrane.

There are probably other ways to do it.