What a Heated Floor Involves Underneath
Electric Mats Are the Practical Option
In a single room retrofit, a thin electric mat set into the tile adhesive is the realistic choice. Hydronic systems make sense when a whole floor plate is being built, not when one bathroom is being redone, because the manifold, the pump and the connection to the heat source outweigh the benefit in a room this size.
It Is a Comfort System, Not a Heating System
Underfloor warmth in a bathroom raises surface temperature and makes the room feel far better. Sizing it as the sole heat source for the space is a different calculation involving insulation and heat loss, and in most existing houses it will not carry the load alone on a cold morning.
Build Up and Thresholds
The mat, its levelling layer and the tile add height. Where the bathroom meets a hallway that difference has to be resolved at the threshold, and where a door swings inward the leaf may need trimming. Small details, but ones that are cheap to plan and irritating to fix after the fact.
The Thermostat Is the Part That Fails
Mats themselves rarely fail if they were tested before, during and after tiling, which is three separate resistance checks and a serious installer will record all three. Thermostats do fail, so mount one where it can be replaced easily and keep a note of the model.
Do Not Cover the Fixed Areas
Mats are not laid under the vanity, under a freestanding tub or where a toilet pan sits, both because the heat is wasted and because trapped heat under a solid object is how an element cooks itself. That means the layout has to be settled before the mat is ordered.