Of everything you can put into a bathroom refurbishment, underfloor heating gives back the most per pound spent. A warm stone or porcelain floor changes how the room feels in a way that a more expensive tap does not. It also frees a wall, because a floor that heats the room means the towel rail can be sized for towels rather than for heat output.
It is also the element most often installed badly, because everything that determines whether it works is buried before anyone sees it. Once the tile is down, the decisions are made. This page covers what those decisions are, in the order they get taken, with the Central London complications that come with mansion blocks, conversions and leases.
Two systems, and they are not interchangeable
Electric
A heating cable or mat laid into the adhesive bed or a levelling compound under the floor finish, run from a fused spur to a thermostat with a floor sensor. Thin — often only a few millimetres of build-up beyond the levelling layer. Quick to install. Independent of the boiler entirely, which matters if the boiler is communal or elderly. Zoned by definition, because it only exists where you put it.
Its limitation is running cost. It converts electricity to heat directly, and electricity is the expensive unit. That is usually fine in a bathroom, because the area is small and it runs on a timer for a couple of short periods a day rather than continuously. It is not a sensible primary heat source for a large area. In a small London bathroom it very often is the right answer, and the reason is the build-up.
Water
Pipe circuits fed from the heating system through a manifold and a blending valve, embedded in screed. Cheaper to run, better suited to heating a room properly rather than warming a surface, and the sensible choice when it is part of a whole-flat system. But it wants depth: a screed build-up, plus the manifold has to live somewhere accessible, plus it has to connect to a heat source that is yours to connect to. Screed also needs real drying time before a finish goes on it — that lands in the programme, and it cannot be argued with. See how long a bathroom takes for where that sits.
In a flat with a communal heating system, or a combi that is already working hard, water underfloor heating in one bathroom is often more trouble than it earns. In a whole-flat refurbishment where the floors are coming up anyway and the heating is being reworked, it makes much more sense.
The decision is your floor build-up
Ask this before anything else: how many millimetres do I have between my structural floor and a finished floor level I can live with?
The answer sets everything. Underfloor heating is not just the mat or the pipe. Done properly it is insulation board, then the heating layer, then levelling or screed, then adhesive, then tile. Each of those is a real thickness, and they stack.
Then the consequences of raising a floor: the door has to clear it, the threshold has to work, and in an older flat with a hall floor at a fixed level you can end up with a step into the bathroom that you did not want. In a mansion block with a concrete slab you are almost certainly building up rather than digging down, because cutting into a structural slab is not a decision that is yours alone to take. In a period conversion with timber joists, you have a little more latitude to work within the floor, but notching joists has structural limits and the plumbing is often already using the depth you were hoping for.
The insulation board is not optional
This is the single most common false economy. Without insulation below the heating layer, a large share of your heat goes downwards — into a concrete slab that is an excellent heat sink, or into the flat below who did not order it. The floor takes much longer to warm up, the thermostat runs longer to get there, and the running cost climbs for a worse result. Insulation board adds a few millimetres of build-up and pays for itself in responsiveness alone. If someone proposes underfloor heating with no insulation to save depth, that is a signal about the rest of the job.
Regulations and certification
Electric underfloor heating in a bathroom is electrical work in a special location. Electrical work in dwellings falls under Building Regulations Part P, and installations are to BS 7671, which sets out the zoning rules that govern what can go where in a bathroom. In practice the thermostat does not belong in the bathroom zones unless it is specifically suitable — commonly it sits outside the room, or a suitable unit is used and positioned properly. The circuit needs the right protection and the installation needs certifying, and you should be handed that paperwork at the end. Bathrooms also engage Part F for ventilation and Part G for sanitation and hot water; our bathroom regulations guide sets out the picture, and our electrical work starts from £80 and is carried out to BS 7671.
Landlords have a further obligation worth remembering: rented homes in England require an EICR at least every five years under the Electrical Safety Standards in the Private Rented Sector (England) Regulations 2020. New work in a rental flat should be documented so it does not become a finding at the next inspection.
The leasehold and building layer
Most leases require freeholder or managing agent consent for alterations, and the triggers commonly include changing floor finishes or floor build-up — which is exactly what underfloor heating does. Some leases also carry acoustic requirements for floors in flats, which interacts with what you build up. Many freeholds in Central London sit with large estates whose licence-to-alter requirements vary considerably, so check your lease and ask your managing agent rather than assume. Our consent guide and licence to alter guide cover it, and in listed stock internal works affecting special character generally need listed building consent.
There is also the question of what the circuit is coming from. In an older mansion block flat, the consumer unit and the incoming supply may not have been designed for the loads a modern flat carries. Adding a circuit is a good moment to look at what is actually there rather than hope.
What the floor finish does to it
Tile and stone are the natural partners: dense, conductive, happy with the heat. Porcelain and marble both work well. Marble is a natural material with its own movement characteristics, and the fixing specification matters more with it than with porcelain — our marble versus porcelain guide goes into that.
Timber over underfloor heating is a specification exercise, not a free choice — engineered board only, within a temperature limit, with the manufacturer's requirements followed. In a bathroom it is rarely the right answer anyway. Our wood flooring page is from £30/m² and covers where it belongs. Vinyl and LVT have their own temperature limits. Whatever the finish, the adhesive and the uncoupling or decoupling layer have to be specified for a heated floor — see tiling, from £55/m².
Controls, and the part people get wrong at the end
A heated floor is only as good as the thing switching it on. The commonest disappointment is not a bad installation — it is a floor set up as though it were a radiator. Underfloor heating is slow. A tiled floor over insulation takes time to come up to temperature and holds heat for a while afterwards, which means it wants a timer that anticipates rather than a switch you press when you are cold. Set to come on before you get up and before you get home, and it feels like magic. Left on a manual switch, it feels like a disappointment you paid for.
Two settings are worth understanding. The floor sensor limits the surface temperature — that is what protects the finish, particularly with natural stone or engineered timber, and it should be set to the finish manufacturer's limit rather than to whatever the box arrived with. The room thermostat governs comfort. A controller that only has one of those is a controller doing half the job. If the flat has other smart heating, it is worth checking at design stage whether the underfloor controller can join it, because retrofitting that after the wall is made good is a nuisance.
Getting it right on site
- Test the cable before, during and after. Insulation resistance readings at each stage, recorded. A cable damaged by a trowel is found in ten minutes now or in a demolished floor later.
- Set the sensor properly. In a conduit, between the runs, not touching them, and reachable — sensors fail more often than cables, and a sensor you can pull out is a repair rather than a rebuild.
- Never cut the heating cable. It is not a length of flex. Set out the mat to the room before it goes down.
- Keep it clear of the WC pan fixings and the shower drain. Someone will drill that floor one day.
- Do not fire it up early. Adhesives and screeds have curing periods and a fast first heat cracks things.
- Commission it with the client present. Show them the controller and hand over the paperwork.
How we do it
We look at the build-up first, tell you which system your floor actually supports, and price it as part of the room rather than as an upsell. Bathroom refurbishment starts from £2,800, and every job is fixed-quoted against a defined scope before work begins. One point of contact, our own trades sequenced in the right order, protection down from the first day, and the flat left clean. Send photographs of the room and, if you know it, what is under the floor.