The the total thickness of the stone+mortar bed will be part of the equation as to how much power will be needed to raise the temperature of the floor. To get this to work without sucking all the money out of your bank account for the electricity bills, you should be working with an insulated slab. That should have been done BEFORE the concrete slab was poured. If you don't have it, then you will want to ADD IT to the install.

Adding insulation material UNDERNEATH the stone will add HEIGHT to the floor. The stone+ mortar can add as much as 1" of height (depends on the thickness of the stone). Adding the insulation + heating mats could add another 1/2" - 3/4" (again it depends on thickness of both materials.

Worse case scenario, you could be looking at 1.75 inches of floor height raise. Which means a LOT of things need to change...like stairs. Codes for stairs are EXTREMELY strict. The run and rise might need to be recalculated. Then there are the door frames, baseboard trim, etc.

If this is a new build with only basic framing and a "temporary workman's stair" in place, then these concerns shouldn't be too much of a bother. If things are already in place, these could be terribly problematic.

Talk to your local mason who has experience doing just what you plan on doing = stone + infloor radiant heat + insulation. See how much his/her AVERAGE installation thickness rates at. I would be astonished if it is less than 1.5 inches.