Underfloor Heating in Extensions London 2026: Costs, Types & Building Regs
Underfloor heating is the ideal companion for a new extension. Here is everything you need to know about choosing between wet and electric systems, meeting Part L requirements, and getting the installation right first time.
Quick Answer
Underfloor heating costs £25-60/m² depending on system type. Wet systems (water pipes) cost more upfront but are cheaper to run. Electric mats are faster to install but cost more to operate. Part L 2021 makes UFH an excellent choice as it works efficiently at low flow temperatures.
£40-60/m²
Wet UFH cost
£25-40/m²
Electric UFH cost
Up to 25%
Running cost saving
Check your specific property constraints
Free Property CheckWhy Underfloor Heating Works Perfectly in Extensions
Extensions are the ideal opportunity to install underfloor heating. You are already laying a new floor slab, which means the additional cost and disruption is minimal compared to retrofitting into an existing room.
With Part L 2021 raising insulation standards, modern extensions retain heat far better than older parts of your home. This makes underfloor heating particularly effective because it operates at lower temperatures and distributes warmth evenly across the entire floor area.
- Part L compliance: UFH works at 35-45°C flow temperatures, making it ideal for heat pumps and helping meet 2021 energy targets
- Frees wall space: No radiators means more room for furniture, glazing, and kitchen units in open-plan layouts
- Even heat distribution: Eliminates cold spots common with radiators, especially in large open-plan kitchen-diners
- Future-proof: Works efficiently with heat pumps, making the transition away from gas boilers easier when the time comes
- Child-friendly: No hot radiator surfaces, no sharp edges, and warm floors for children playing at ground level
Types of Underfloor Heating for Extensions
There are two main types of underfloor heating, each suited to different situations. For new-build extensions, wet systems are almost always the better long-term choice. Electric systems have their place in smaller spaces or where floor buildup is limited.
Wet (Water) Underfloor Heating
Wet systems circulate warm water through a network of cross-linked polyethylene (PEX) pipes embedded in or laid on top of the floor. The pipes connect to your boiler, heat pump, or a dedicated heat source via a manifold that controls flow to different zones.
Cost: £40-60/m² installed
Best for new-build extensions where you are already laying a floor slab
Advantages
- Low running costs (up to 25% less than radiators)
- Works at low flow temperatures (35-45°C) with heat pumps
- Long lifespan (50+ years for pipes)
- Heats large areas efficiently and evenly
Disadvantages
- Higher installation cost than electric
- Requires floor buildup of 60-80mm (screed over pipes)
- Slower heat-up time (2-3 hours from cold)
- Screed needs 4-6 weeks drying time before flooring
Electric Underfloor Heating
Electric systems use thin heating mats or loose cables installed directly beneath the floor finish. They plug into the mains electrical supply and are controlled by a thermostat. The heating elements sit in flexible adhesive or self-levelling compound, adding minimal height to the floor.
Cost: £25-40/m² installed
Best for small areas, bathrooms, or where floor buildup is limited
Advantages
- Ultra-thin profile (3-5mm buildup)
- Quick installation (1 day for most rooms)
- Lower upfront cost per square metre
- Fast heat-up time (20-30 minutes)
Disadvantages
- Higher running costs (electricity vs gas/heat pump)
- Not ideal for whole-house or large area heating
- Shorter lifespan (15-25 years typical)
- Cannot connect to heat pumps
Wet vs Electric: Side-by-Side Comparison
| Feature | Wet (Water) UFH | Electric UFH |
|---|---|---|
| Install cost per m² | £40-60 | £25-40 |
| Running cost per m²/year | £8-12 | £15-22 |
| Floor buildup | 60-80mm (with screed) | 3-5mm (mats in adhesive) |
| Heat-up time | 2-3 hours from cold | 20-30 minutes |
| Lifespan | 50+ years | 15-25 years |
| Heat pump compatible | Yes (ideal pairing) | No |
| Best for | New extensions, large areas, whole-house | Small areas, bathrooms, retrofits |
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Building Regulations for UFH in Extensions (2025/2026)
Part L 2021 sets strict energy efficiency targets for new extensions. Underfloor heating helps meet these targets because it operates at lower temperatures than radiators, reducing energy demand. However, the system must be properly specified and the floor construction must be designed to work with it.
Part L Key Requirements for Heated Floors
The floor U-value must not exceed 0.18 W/m²K. With underfloor heating, insulation must be placed below the heating element to prevent heat loss downwards. At least 100mm of PIR insulation is typically required beneath UFH pipes or mats.
- SAP calculations: Your designer must demonstrate the extension meets energy targets. UFH at low flow temperatures improves the SAP score significantly
- Insulation below UFH: Minimum 100mm PIR board (or equivalent) beneath pipes to direct heat upwards into the room
- Heat loss calculations: Each room must be individually assessed to determine pipe spacing and flow rates for adequate heating
- Zone controls: Building regulations require independent temperature control for each zone, typically achieved via the UFH manifold and room thermostats
- Commissioning: The system must be pressure-tested and commissioned by a qualified installer before the building control sign-off
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Ask our AI for a free recommendation →Best Flooring for UFH in Extensions
Not all floor finishes work well with underfloor heating. The key factor is thermal conductivity: you want a material that allows heat to pass through efficiently rather than acting as an insulating blanket on top of the heating element.
| Flooring Type | UFH Suitability | Notes |
|---|---|---|
| Porcelain tile | Excellent | Best conductor, heats quickly, retains warmth |
| Natural stone | Excellent | Great thermal mass, feels warm underfoot |
| Polished concrete | Excellent | Seamless look, excellent thermal mass, very popular in modern extensions |
| Engineered wood | Good | Must be max 18mm thick, use floating or glued installation |
| LVT (luxury vinyl tile) | Good | Thin profile helps conductivity, good moisture resistance |
| Solid hardwood | Poor | Risk of warping, gaps, and cupping with heat cycles |
| Thick carpet | Poor | Insulates against heat, max 1.5 tog rating if used |
Avoid Solid Wood and Thick Carpet
Solid hardwood expands and contracts with temperature changes, leading to gaps, cupping, and potential cracking over time. Thick carpet acts as an insulating layer, forcing the system to work harder and increasing running costs. If you want carpet in a bedroom area, keep the total tog rating (carpet plus underlay) below 1.5.
UFH Cost Breakdown by Extension Size
The following costs include materials, labour, manifold, controls, and commissioning. Wet system costs assume connection to an existing boiler. Heat pump installation is additional.
| Extension Size | Wet UFH Cost | Electric UFH Cost | Annual Running (Wet) |
|---|---|---|---|
| 15m² | £600-900 | £375-600 | £120-180 |
| 20m² | £800-1,200 | £500-800 | £160-240 |
| 30m² | £1,200-1,800 | £750-1,200 | £240-360 |
| 40m² | £1,600-2,400 | £1,000-1,600 | £320-480 |
Prices include manifold, thermostat, pipes/mats, and labour. Screed and floor finish are separate. Add £500-1,000 for smart thermostat controls.
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Installation Considerations
Getting the installation right is critical. Underfloor heating is buried beneath your floor finish, so mistakes are expensive and disruptive to correct. Here are the key things to get right during the build.
Screed Drying Time
Wet UFH systems require liquid screed to be poured over the pipes. This screed must dry completely before any floor finish is applied. Allow 4-6 weeks minimum drying time at 1mm per day for the first 40mm, then 0.5mm per day thereafter. Rushing this step causes cracking, debonding, and moisture damage to floor finishes.
Zone Control
Divide your extension into separate heating zones via the manifold. At minimum, separate the kitchen area from any living or dining space. This allows different temperatures in different areas and prevents overheating in zones with high solar gain from glazing.
Thermostat Placement
Room thermostats should be positioned 1.5m from the floor on an internal wall, away from direct sunlight, draughts, and heat sources. Floor probes embedded in the screed provide additional control and prevent floor surface overheating above 27°C (the safe maximum for most floor finishes).
Commissioning
The system must be pressure-tested at 6 bar for 24 hours before screed is poured. After screed has cured, a gradual warm-up over 2-3 weeks is essential: start at 20°C and increase by 5°C per day until reaching operating temperature. This prevents thermal shock to the screed.
Common Mistakes to Avoid
We see these issues regularly on London extension projects. Each one is avoidable with proper planning.
- Not enough insulation below: Skimping on insulation beneath the pipes sends heat downwards into the ground. You need at least 100mm PIR board, ideally 150mm for ground-floor extensions
- Choosing the wrong flooring: Solid hardwood and thick carpet significantly reduce system efficiency. Discuss your floor finish with the UFH installer before ordering materials
- No zone control: A single thermostat for the entire extension means you cannot adjust temperatures in different areas. Kitchen islands and south-facing glazing create hot spots that need independent control
- Mixing UFH with radiators incorrectly: If the extension has UFH while the rest of the house uses radiators, a mixing valve is essential. Radiators need water at 60-70°C while UFH needs 35-45°C. Running them at the same temperature means one system will underperform
- Rushing the screed cure: Turning on UFH before the screed has fully cured causes cracking and compromises the floor structure. Patience during the 4-6 week drying period saves costly remedial work later
Frequently Asked Questions
Frequently Asked Questions
How much does underfloor heating cost in a London extension?
Wet underfloor heating costs £40-60 per square metre installed, while electric systems cost £25-40 per square metre. For a typical 20m² extension, expect to pay £800-1,200 for wet UFH or £500-800 for electric. These costs include pipes or mats, manifold, thermostat, and labour but exclude screed and floor finish.
Is wet or electric underfloor heating better for an extension?
Wet UFH is almost always better for new extensions. You are already laying a new floor slab, so the installation is straightforward. Running costs are significantly lower, lifespan is longer, and it works with heat pumps for future energy efficiency. Electric is only better for very small areas like a single bathroom.
Can I use underfloor heating with a heat pump?
Yes, and it is an excellent combination. Heat pumps are most efficient when delivering water at 35-45°C, which is exactly the temperature range underfloor heating requires. This pairing typically delivers a coefficient of performance (COP) of 3-4, meaning 3-4 units of heat for every unit of electricity used.
How long does underfloor heating take to heat up?
Wet underfloor heating takes 2-3 hours to warm a room from cold. Electric systems heat up in 20-30 minutes. However, most people leave UFH running at a low background temperature and use a thermostat to boost when needed, so you rarely experience the full heat-up time.
Do I need building regulations approval for underfloor heating?
UFH itself doesn't require separate building regulations approval, but your extension does. The heating system must comply with Part L (energy efficiency) and the floor construction must meet U-value requirements. Your building control officer will check the UFH specification as part of the overall extension sign-off.
Can I have underfloor heating under wooden floors?
Engineered wood up to 18mm thick works well with UFH. Avoid solid hardwood as it warps and gaps with temperature changes. Choose a species with good dimensional stability like oak, and use either floating or glued-down installation. Never nail down flooring over UFH pipes.
Summary
Underfloor heating is the best heating choice for a new London extension. Wet systems offer lower running costs and future-proof compatibility with heat pumps, while electric systems suit smaller supplementary areas. The key is getting the specification right: adequate insulation below, compatible flooring above, proper zone controls, and patience during the screed curing process.
With Part L 2021 demanding higher energy efficiency standards, underfloor heating helps your extension comply while delivering superior comfort compared to radiators. The additional cost over radiators is typically £500-2,000 for a standard extension, but the long-term savings and comfort benefits make it a worthwhile investment.
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