Underfloor Heating in a London Extension: Wet or Electric?

Underfloor heating is one of the few extension decisions that has to be right before the floor is closed. Once the screed has cured or the boards are down, the layout beneath, the insulation and the finished level are fixed with it. So the wet-or-electric question is worth answering early, alongside the heat source, the floor structure and the kitchen layout.

The short answer for a London extension

Neither wet nor electric underfloor heating is universally better. Anyone who says otherwise is usually describing a product range rather than your room.

For a larger extension that will be heated frequently — a kitchen-dining space, a family room, an area open to the rest of the house — a properly designed wet system often deserves serious consideration. It can work with a range of heat sources, and across larger, regularly heated areas it may cost less to run than direct electric. Test that with your designer rather than treating it as a promise.

For a small or intermittently heated zone — a shower room, a cloakroom, a utility, or anywhere running pipework and a manifold across the house would be disproportionate — electric may well be the practical answer. A simpler installation is a genuine advantage, though not the same thing as a cheaper bill.

The outcome depends on the room's heat loss, the heat source, your tariff, the controls, how often the space is actually warm, the floor structure and available build-up, and the finish. Change one and the sensible answer can change with it.

Decide from a room-by-room heat-loss calculation, not floor area

The most common mistake is picking an output from floor area. Heat demand comes from fabric and exposure: glazing, insulation in the walls and roof, floor construction, ventilation, and how the new space connects to the older part of the house.

At the time of writing (September 2026), Approved Document L Volume 1, 2021 edition incorporating 2023 amendments, is the current principal Part L guidance in England. It says a space-heating specification should be based on an appropriate heat-loss calculation and an appropriate sizing methodology, and that systems should not be significantly oversized. An Approved Document is guidance on one way of meeting the requirements rather than the law itself, and it is not the only route to compliance. A 2026 edition has been published, but it takes effect on 24 March 2027 for building work not connected with higher-risk building work and on 24 September 2027 for higher-risk building work, subject to transitional arrangements, so confirm which edition applies to your project's date.

The calculation also tells you whether the floor alone can meet the demand. In a heavily glazed rear extension it sometimes cannot, and the design then needs a supplementary emitter or a fabric improvement. Far better to find that on paper. Our house extension pre-start checklist sets out where this sits in the sequence of pre-start decisions.

Wet and electric underfloor heating in plain English

A wet system circulates warm water through pipework laid within or beneath the floor. Pipes are grouped into circuits and connected to a manifold, fed by a heat source such as a boiler or heat pump, and controlled through actuators, a pump and a blending arrangement that governs the temperature entering the floor. It behaves as part of your central heating and has to be designed as part of it.

A direct electric system heats the floor using cable or matting laid under the finish, connected to a protected electrical supply and controlled by a thermostat with a floor and/or air sensor. No water, no manifold, no hydraulic balancing. The Energy Saving Trust draws the same basic distinction between electric wires beneath or within the floor and water-filled pipes served by a boiler or heat pump.

One caution: electric underfloor heating is not a single product. Approved Document L distinguishes direct-acting electric systems from electric storage systems, with different control and sensing considerations. Follow the specification and manufacturer's instructions for the system you are actually buying.

Installation complexity against operating context

The two options load their cost differently, and it helps to keep installation and operation apart.

Electric is usually the simpler build: a thinner heated layer, no manifold, no pipework running back to a plant position, less coordination with the heating engineer. Wet systems bring more trades and more sequencing — pipework, manifold location, screed or overlay, and the connection into the heat source — and they need a design before anything is laid.

Operation often reverses the picture. The Energy Saving Trust notes that direct electric underfloor heating is commonly used for smaller or intermittently heated areas, while wet systems can have lower running costs across larger, more frequently heated ones. That is decision context, not a prediction. Your bills will depend on tariff and fuel prices, the heat source and its efficiency, the room's heat loss and insulation, operating temperature and controls, the heated area and how you use it, the floor finish and build-up, and the weather.

Nobody can give you a credible percentage saving or payback period without those figures. What you can reasonably ask for is a comparison of the options against your calculated heat loss and your realistic pattern of use.

Wet systems: heat source, design temperatures and building control

A wet floor is only as good as the heat source behind it, and the two decisions have to be taken together.

The central question is the temperature the system is designed to run at. Underfloor heating generally operates at lower flow temperatures than radiators, which is one reason it sits comfortably with a heat pump, but the specific design temperatures come from your heat-loss calculation, the floor construction and the emitter design, not from a standard figure. If a heat pump is in the picture now or might follow later, say so at design stage: it affects pipe layout, floor construction and the emitter design elsewhere in the house.

Where a wet floor shares a heat source with radiators, the blending and control arrangement matters, and so does zoning. Approved Document L addresses thermostatic room controls and the circumstances in which adjacent rooms or open-plan areas may reasonably form one heating zone; your designer should apply that to your actual layout rather than a generic one.

The Planning Portal's underfloor heating guidance says wet systems must comply with Part L and will likely require building control approval. In an extension the heating will normally sit within the wider Building Regulations work already under way, but that is not a universal statement about every standalone alteration. The Planning Portal's heating and building regulations page is a useful starting point, and your building control body will confirm the route for your project.

Electric systems: Part P, circuits and the notification question

Electric underfloor heating is electrical installation work and sits within Part P of the Building Regulations. Approved Document P states that all electrical installation work in a dwelling is subject to the Part P requirement, whether or not a particular job has to be notified.

Notification is a separate question, and it is frequently reported badly. Approved Document P identifies notifiable work as the installation of a new circuit, the replacement of a consumer unit, or additions and alterations to existing circuits in a special location, such as a room containing a bath or shower. Fixed electrical equipment falls within the scope of Part P, but whether a given installation is notifiable depends on whether the work falls within that defined scope. So the work must comply; whether it must be notified depends on what is being done and where.

Practically, the consumer unit deserves attention early. An extension often adds circuits, and the existing board may not have the capacity or the protective arrangements to take them. That conversation belongs while first fix is still open, not after the floor is finished. Ask for certification of the completed electrical work by a competent person and keep it in your project file.

The floor build-up: structure, insulation, moisture and finished level

Underfloor heating is a layered assembly, and every layer needs deciding before anything is laid.

Your floor structure sets the options. The Planning Portal's guidance on extension flooring notes that floors have to deal with structural support, resistance to ground moisture and heat loss, and that solid floors, suspended timber floors and suspended concrete floors are built up differently. A ground-bearing slab in a new rear extension is not the same problem as a suspended timber floor in the existing house, and the two frequently meet in the middle of one open-plan room.

Insulation beneath the heated layer matters more here than in an unheated floor, because heat travels downwards as readily as upwards. Approved Document L deals with limiting heat loss through ground and intermediate floors and with insulating distribution pipework. There is no single correct thickness; it comes from the design for your floor.

Then there is the arithmetic that catches people out: total build-up and finished floor level. Structure, insulation, any moisture layer, the heating layer, screed or overlay and the finish all stack up. If the finished level is settled late, you end up trimming doors, losing height at the threshold, or finding the new floor no longer lines through with the old. Fix the finished level on a drawing early and work backwards to what each layer can be.

Thresholds, doors, stairs and the junction with the existing house

The finished level decision does not stop at the room. It travels to every edge.

Thresholds to the garden are the obvious case. A level threshold, a shallow step and a full step each demand different decisions about drainage, the door system and the build-up behind it, and the heating layers form part of that stack. Resolve that trade-off during the extension design and build sequence, before the door and floor specifications are fixed.

Inside, check door swings and undercuts, the junction where the new floor meets the old, the bottom step of any staircase within the works, and the skirting detail. A floor that has risen even modestly changes how skirtings sit, whether internal doors need cutting down and whether an existing threshold strip still works. Where new meets old, decide in advance whether you are levelling the old floor up, forming a deliberate transition or accepting a trim, and whether the heated area stops at that line or carries through it. None of this is expensive to resolve on a drawing. All of it is expensive after the screed has cured.

Fixed furniture, no-drill zones and choosing the floor finish

Before anything is installed, mark up where heat is not needed and where nothing may be fixed into the floor.

In a kitchen that usually means the run of base units and appliances: heating a floor permanently covered by cabinetry rarely earns its place, and some manufacturers restrict it. Islands carry the bigger risk, because they are often fixed down through the floor, and an island that moves after the pipes or cables are set can turn a fixing hole into a repair. The same applies to bathroom furniture, WC and basin fixings, and any upright fixed into the slab. Agree those positions before the floor is laid; our guide to what to expect during a kitchen renovation shows where that decision sits in the programme. Ask for an as-installed layout drawing, or photographs taken before covering, so anyone drilling into that floor in future knows where not to.

The floor finish is often chosen last, but it constrains everything. Different finishes have different thermal resistance, and one that insulates the heat away can undermine the system. Check compatibility — the specific finish, its thickness, the adhesive or underlay, and the maximum permitted floor temperature — with your supplier and system designer before you buy.

Zoning, controls, commissioning and what you are handed at the end

Two rooms with identical floors can produce very different bills, and the difference is usually control and use.

Decide honestly how each space will be run. A kitchen-dining room warm from morning to evening behaves quite differently from a shower room heated for an hour a day, and that difference sits at the centre of the wet-or-electric decision. Approved Document L expects new underfloor heating systems to have controls that adjust the operating temperature, and says room thermostats for electric systems should have a manual override. Get the zoning drawn: which areas are separately controlled, where each thermostat and sensor sits, and how the system interacts with the rest of the heating.

Then agree testing and commissioning before the floor is covered, because afterwards the options narrow sharply. Approved Document L says fixed building services should be tested and adjusted as necessary and in accordance with manufacturer instructions, and that installed underfloor heating equipment should be commissioned in accordance with BS EN 1264-4. The specific tests, values and durations come from your system specification and the manufacturer. What you should insist on is that they happen, that they are recorded, and that the record reaches you.

At handover, ask for the as-installed layout, commissioning and test records, electrical certification where applicable, the control settings and the manufacturer's operating instructions, and keep them with your building control paperwork. Years later, when someone wants to change the floor covering or drill a fixing, that file is the only thing standing between them and the heating.

Before the floor is closed, work through the following with your designer, heating engineer and electrician.

1. Commission a room-by-room heat-loss calculation for the extension before choosing between wet and electric.
2. Confirm the heat source and the design temperatures for a wet system, including whether a heat pump is planned now or may follow later.
3. Ask your electrician to assess consumer-unit and circuit capacity, Part P compliance and whether your specific scope of work is notifiable.
4. Confirm the building control route for the heating and the extension with your building control body rather than assuming it.
5. Fix the finished floor level and the full build-up — structure, insulation, moisture layer, heating layer, screed or overlay and finish — on a drawing.
6. Resolve thresholds, door undercuts, stair junctions, skirtings and the transition to the existing floor at design stage.
7. Mark heated and unheated areas, including beneath fixed kitchen and bathroom furniture, before installation begins.
8. Agree island and fixed-furniture positions and no-drill zones, and record them on an as-installed layout.
9. Check the chosen floor finish for system compatibility, thermal resistance and maximum floor temperature with the supplier and designer.
10. Agree zoning, thermostat and sensor positions, testing, commissioning and the handover pack before the floor is covered.

None of this removes the wet-or-electric question. It means you answer it from your own numbers rather than someone else's product range. Being in London does not change that: the applicable edition of the guidance, the building control route and the property-specific requirements all still need checking for your project and its date before work starts.