Underfloor Heating for London Extensions and Refurbishments 2025: Wet, Dry, and Costs

**The two fundamentally different types of underfloor heating**:

**Wet (hydronic) UFH**: water is heated by a boiler or heat pump, pumped through a manifold, and circulated through plastic pipes (typically PEX-a or PEX-b; 16mm or 20mm diameter) embedded in a floor screed or routed between joists. The water temperature is controlled at the manifold.

**Dry electric UFH**: electrical resistance cables or mats are embedded in tile adhesive or a thin overlay below the floor finish. The system generates heat directly from electricity — no water, no pipes, no manifold, no connection to the heating system.

**When to use wet UFH**:

• *For a new single-storey rear extension*: wet UFH in liquid screed is almost always the preferred specification. The reasons:
• •The extension floor build-up starts from a new structural slab or oversite — there is no existing floor to disrupt; the screed can be designed into the floor build-up from the outset
• •If the house has or will have an ASHP, the extension UFH is the natural low-temperature emitter to connect to the heat pump — maximising heat pump efficiency
• •The running cost of wet UFH connected to an ASHP is significantly lower than electric UFH — electricity used for resistance heating (electric mat) is used at 1:1 efficiency; electricity used to run an ASHP (which moves heat rather than generating it) achieves a COP (Coefficient of Performance) of 2.5–4.5x — each unit of electricity moves 2.5–4.5 units of heat
• •The pipe spacing and manifold design can be set for either high-temperature (gas boiler) or low-temperature (ASHP) operation in the original specification, future-proofing the system

*For a full-house refurbishment with a concrete ground floor*: wet UFH in a liquid screed overlay is feasible where the existing floor level can accommodate the screed build-up (minimum 65mm liquid screed above the pipe; total floor build-up from the existing concrete slab: approximately 100–130mm from slab surface to finished floor level). This is only possible where the existing floor level allows for the build-up without creating an excessive step at doors or compromising ceiling height.

*For a ground-floor refurbishment with timber suspended floor (original Victorian joists)*: wet UFH between joists (warm water joist system) is possible — see below for the specific products and constraints.

**When to use dry electric UFH**:

*For a bathroom or kitchen floor tile*: electric UFH mat (a thin resistance cable bonded to a glass fibre mat, typically 3.5–6mm total depth) installed in the tile adhesive bed is an excellent low-cost solution for a small area (a bathroom floor: 3–8m²; a kitchen floor: 10–20m²) where the primary goal is comfort (removing the sensation of cold tiles underfoot) rather than space heating. The running cost of electric UFH for a small bathroom is manageable (a 600W mat for a 6m² bathroom floor: approximately £0.15/hour at current UK electricity rates; typically runs 1–2 hours/morning on a timer: approximately £0.10–£0.30/day).

*For retrofit where no screed is possible and no joist system is viable*: where head height prevents a screed overlay and the joist spacing makes a between-joist system uneconomic, an electric overlay system (thin electric mat installed below LVT or engineered timber with a thin levelling compound) is the last resort. Not recommended as a primary heating system — best used as comfort heating supplementary to existing radiators.

*Dry electric UFH should NOT be used as the primary heating system for a large open-plan extension*: the running cost of resistance electric heating for a 20–30m² extension is very high at UK electricity rates (typically 4x higher running cost per kWh than a gas boiler; 2x higher than an ASHP). Size the system as supplementary comfort heating only if electric UFH is chosen for a large area.

**1. Wet UFH in liquid screed — the standard specification for new extensions**:

Liquid screed (also called flowing screed; calcium sulphate screed; or anhydrite screed) is the preferred floor medium for wet UFH in new extensions. The process:

*How it works*: PEX-a UFH pipes are clipped to a polystyrene insulation board (typically 100–150mm rigid EPS or PIR, providing both thermal insulation under the floor and the pipe clip substrate) laid on the structural slab. The manifold (the control unit that distributes water from the boiler/heat pump to each floor zone circuit) is mounted on the wall of the utility room or cupboard. Pipes are run from the manifold in loops, clipped to the insulation at the specified pipe spacing (typically 150mm or 200mm centres, depending on the heat loss calculation for the space), and connected back to the manifold returns. Liquid screed is then pumped across the floor at a depth of typically 65–75mm above the top of the pipes, self-levelling to a flat surface.

• *Key advantages of liquid screed for UFH*:
• •Self-levelling to ±3mm over a 3m bay in a well-prepared substrate — produces a very flat floor surface, excellent for tiles and LVT
• •Faster heat-up time than sand-cement screed (lower thermal mass per mm depth; greater contact with the pipes)
• •Lower drying-time-per-mm than sand-cement (calcium sulphate screed: typically 1 day per mm thickness for the first 40mm, then 0.5 days per mm beyond — a 70mm screed: approximately 55 days to 75% relative humidity, suitable for most floor finishes; 75–90 days to <0.5% moisture for engineered timber)
• •The liquid screed can be poured by a specialist liquid screed company using a machine mixer — fast installation (a 30m² floor: typically 1 day)

*The insulation layer under the UFH screed*:

The insulation under the UFH screed serves two functions: it reduces the U-value of the ground floor (to comply with Part L 2021: 0.18 W/m²K for a new extension ground floor); and it prevents heat from the UFH pipes being lost downwards into the ground rather than upwards into the room.

• Typical insulation specification for extension ground floor UFH:
• •100mm EPS70 (expanded polystyrene; λ = 0.037 W/mK): achieves approximately 0.23 W/m²K (below the Part L limit without additional wall insulation) — needs to be 150mm to achieve 0.18 W/m²K in an uninsulated ground
• •100mm PIR (λ = 0.022 W/mK): achieves approximately 0.18 W/m²K — sufficient for Part L compliance
• •150mm EPS70: achieves approximately 0.18 W/m²K — acceptable for Part L

For new extensions where the floor build-up height is flexible, 150mm PIR + 65mm liquid screed + floor finish is a very good specification for both Part L compliance and UFH efficiency.

**2. Warm water joist system — UFH without screed, between floor joists**:

Where a timber suspended floor (original Victorian ground floor joists) is being refurbished and UFH is desired without the weight or height gain of a liquid screed, a warm water joist system is the alternative. Two common products:

• •**Warmup Foil Joist System**: PEX-a pipes are clipped into aluminium diffuser plates that sit between the joists (notched into the joist if the pipe has to pass through rather than between) — the aluminium diffuser plate spreads the heat from the pipe across the full width of the joist bay. The floor decking (tongue-and-groove; OSB3; plywood) is laid on top of the joists, and the floor finish goes on top of the decking. No screed; minimal floor height gain (typically 10–25mm above the existing joist depth for the diffuser plate and decking)
• •**Heatmiser ClipUp** and similar: functionally similar products from other manufacturers using aluminium diffuser plates between joists

• *Key limitations of the joist UFH system*:
• •Aluminium diffuser plates are effective at spreading heat laterally, but they are less thermally efficient than the full screed-embedment of a liquid screed system — heat output per pipe length is lower, requiring closer pipe spacing or higher flow temperatures to achieve the same heat output
• •The system requires access to the top of the joists to install the diffuser plates and clip the pipes — usually done as part of a floor refurbishment where the existing floor boards are being lifted and replaced. Retrofitting to an existing floor without lifting the boards is not possible
• •Heat output from the system is typically 60–80% of an equivalent screed-embedded system for the same pipe spacing — the floor may not achieve the same comfort level or heat output per m² as a screed system at low ASHP flow temperatures. Specify at 150mm or 100mm pipe spacing to compensate for the lower thermal efficiency
• •Not all floor finishes are compatible — check the manufacturer's maximum board thickness specification for the specific diffuser plate product

**3. Dry overlay systems — retrofit UFH with minimal floor height gain**:

For retrofit situations where the floor cannot be lifted (concrete floor that cannot be raised; tiled floor that will be retained) and a screed is not possible, dry overlay UFH systems provide a thin-profile option:

• •**Warmup Overlay** and similar products: a thin aluminium-faced panel (typically 12–18mm depth) into which PEX pipes sit in pre-routed channels; installed directly over the existing floor, with the floor finish going on top
• •The build-up height added to the existing floor level: typically 20–30mm (panel depth + adhesive + floor finish)
• •The system is suitable for LVT or engineered timber finishes (≤12mm engineered timber above the panel for best heat transfer); not suitable for thick solid timber or heavy tile without checking heat output calculations
• •Heat output is lower than screed-embedded systems — typically only suitable for supplementary comfort heating rather than primary space heating in larger rooms

**4. Manifold, pump station, and control system**:

• Every wet UFH system requires:
• •**A manifold**: the distribution unit that routes hot water from the boiler/heat pump to each floor circuit and back. Manifolds are sized by the number of circuits (zones). For a single-storey extension: typically 2–4 circuits (one per room; one for the open-plan kitchen-living area). Manifold cost: **£300–£700** supply-only depending on size and manufacturer (Polypipe; Emmeti; Honeywell; Uponor)
• •**A pump station** (or pump head): the pump that drives flow through the UFH circuits. For connections to a gas boiler: a blending/mixing unit (thermostatic blending valve) is needed to reduce the boiler flow temperature (typically 65–80°C) down to UFH operating temperature (35–50°C for gas; 30–40°C for ASHP). Cost: **£400–£900** supply-only
• •**Zone thermostats and actuators**: each circuit has a wax actuator on the manifold valve, controlled by a room thermostat. Cost per zone: **£40–£120** for thermostat + actuator
• •**System commissioning**: the completed UFH system must be pressure-tested (3 bar static pressure; 30-minute hold) before screed is poured; and the screed must be heat-commissioned (gradual warm-up protocol: 25°C for 3–4 days; then 40–50°C for 3–4 days; then operating temperature) before floor finishes are installed

**2025 underfloor heating supply and installation costs for London extension and refurbishment projects**:

*Wet UFH in liquid screed — complete system supply and installation (pipes, manifold, pump station, insulation board, liquid screed, pressure test, commissioning)*:

| Area | Pipe spacing | System type | Cost/m² supply + install London | |---|---|---|---| | Up to 20m² extension | 150mm | PEX in liquid screed | £55–£75/m² | | 20–40m² extension | 150–200mm | PEX in liquid screed | £50–£70/m² | | 40–60m² extension or open-plan | 150–200mm | PEX in liquid screed | £45–£65/m² | | Warm water joist system | 100–150mm | Aluminium plate; no screed | £70–£110/m² | | Dry electric mat (tile adhesive) | N/A | Cable mat | £20–£45/m² | | Dry electric overlay (retrofit) | N/A | Thin overlay panel | £40–£70/m² |

*Note: the above costs include the manifold and pump station for wet systems, amortised across the floor area. Small areas (<15m²) have a higher cost/m² because the manifold and pump station fixed cost is spread across fewer m².*

• *For a typical 20m² single-storey rear extension (wet UFH in liquid screed; 2-zone manifold; ASHP-compatible specification)*:
• •Underfloor insulation board (100mm PIR): £600–£900
• •PEX-a pipe supply and installation: £400–£700
• •Manifold and pump station supply and installation: £700–£1,200
• •Liquid screed (65–75mm depth; 20m²): £600–£1,000
• •Pressure testing and commissioning: £200–£400
• •**Total system cost: £2,500–£4,200**

**Floor finish compatibility with wet UFH**:

• *Best floor finishes for UFH*:
• •**Large-format porcelain or stone tile** (600×600mm; 600×1200mm; 900×900mm): the best thermal conductor of all floor finishes; no restriction on tile size or thickness for UFH. The thermal response time with large-format tile over liquid screed is 2–4 hours (the screed and tile absorb and then slowly release heat — this is why UFH works best with programming rather than on-demand switching)
• •**Luxury Vinyl Tile or Plank (LVT)**: fully compatible with UFH; good thermal conductivity; warm feel underfoot even without UFH running. Maximum temperature at the underside of LVT: typically 27°C (check the LVT manufacturer's specification). Most LVT products are rated to 27–28°C subfloor temperature
• •**Engineered timber flooring** (maximum 24mm thickness — thinner boards give better heat transfer): compatible with UFH, but check the manufacturer's specification for maximum flow temperature (typically 50°C for the UFH circuit, which at 150mm pipe spacing produces a floor surface temperature of approximately 26–28°C — within most engineered timber specifications). The moisture content of engineered timber above UFH must be maintained carefully — liquid screed must be fully dried (below 0.5% moisture for most timber finishes, which typically requires 8–12 weeks of air-drying after heat commissioning)

• *Floor finishes to avoid or approach with caution on UFH*:
• •**Solid oak or solid hardwood flooring** (greater than 20mm thickness): solid timber has very poor thermal conductivity and moves significantly with temperature and humidity changes; not recommended for wet UFH. If specified, it must be acclimated carefully and the maximum floor surface temperature must be limited
• •**Carpet with heavy underlay**: the thermal resistance of carpet and underlay significantly reduces heat output — a 10mm wool carpet can reduce heat output by 30–50%. If carpet is desired over UFH, specify a thin, flat-pile carpet (not deep-pile shag) with a thin rubber underlay (not foam underlay) and check the combined tog rating does not exceed 2.5 tog
• •**Solid natural stone more than 20mm thick** (thick slate; natural granite): very good thermal conductor (beneficial for heat transfer) but very high thermal mass — very long heat-up time (4–8 hours) and very slow response to thermostat demand changes

**Part L 2021 compliance for extension UFH**:

The Building Regulations Part L (Conservation of Fuel and Power) for new extensions does not specifically mandate UFH — it specifies maximum U-values for the building fabric (floor: 0.18 W/m²K; walls: 0.18 W/m²K; roof: 0.15 W/m²K) and notional heating system performance. An extension with UFH must meet the same fabric U-values as any other extension.

*Where UFH specifically creates a Part L compliance benefit*:

When the extension UFH is connected to an ASHP as the heat source (rather than a gas boiler), the carbon and energy performance of the heating system is significantly better — the ASHP achieves a seasonal COP of 2.5–3.5+ in the UK climate, while a condensing gas boiler achieves 85–95% efficiency. The Part L SAP calculation (the energy performance calculation for a new extension on an existing house) reflects this heating system difference. An ASHP + UFH combination can enable compliance with Part L 2021 for extension projects that would be more difficult to achieve with higher-carbon heating.