Would a self leveller be required before tiling over a electric heating matt

No its not required. Some suggest it with a wire purely to prevent the tiler damaging the wires but its an optional step and even more optional with a mat

I don’t really subscribe to the view that an SLC over a heater mat “prevents the tiler damaging it” any more than installing capping over electrical cable drops prevents “clumsy plasterers” damaging them. You can tile straight over a heater mat but it’s worth considering it will be impossible to repair if it ever goes faulty; you will never get the tiles up without destroying the mat! Maybe useful, maybe not.


Edit; should have said that without an SLC, you might find it more difficult to tile & keep the finished floor flat if you’re inexperienced & it will be even more of a challenge for you if using large format tiles, you will also use slightly more tile adhesive.