Heating an Extension — Options, Costs, and What Works Best

Underfloor heating (UFH) is the most popular choice for single-storey rear extensions in London, particularly kitchen-diner extensions with large-format tiles or stone flooring.

**How it works**: Wet UFH consists of a network of small-bore pipes embedded in the floor screed or within a low-profile floor system. Hot water circulates through the pipes from the boiler (or heat pump) via a manifold. The floor surface temperature is typically 26–30°C — below bare-foot comfort threshold — and the radiant heat from the floor heats the room evenly from the ground up.

• **Why it works well in extensions**:
• •Extensions with large-format stone or porcelain tiles (a very common finish) benefit particularly from UFH — tiles are excellent thermal conductors and spread the heat well
• •An open-plan kitchen-diner extension has no obvious place for large radiators — UFH removes the need for radiators entirely, freeing the walls for units, glazing, and doors
• •The even heat distribution is comfortable in large open spaces where a single radiator would create hot spots
• •UFH runs at lower water temperatures than radiators (typically 35–45°C vs 70–80°C) — more efficient with a condensing gas boiler and very efficient with a heat pump

**Two types of wet UFH system**: 1. **Screed system**: pipes are embedded in a liquid or sand/cement screed, typically 65–100mm deep. The screed is the floor finish substrate. Requires 65–100mm floor depth — sometimes a constraint in extensions where floor levels need to meet existing house levels. 2. **Low-profile overlay system**: pipes run within a 15–25mm proprietary board system, which can be overlaid directly onto the existing floor or a new insulated base. Less depth needed, faster response time, but more expensive per m².

• **UFH costs for a 25m² extension kitchen-diner (London, 2025)**:
• •Wet screed system (pipes + screed + manifold): £3,500–£6,500
• •Low-profile overlay system: £4,500–£8,000
• •Both require a connection to the boiler and a zone control (thermostat and actuator): typically included in the above or an additional £300–£600

• Standard radiators remain a valid choice for extensions — particularly where:
• •The extension has engineered wood or LVT flooring (neither is ideal with underfloor heating without specific low-temperature boards)
• •The floor depth cannot accommodate a UFH system without raising floor levels
• •Budget is a constraint
• •The extension is relatively small (under 15m²) where a single well-placed radiator is sufficient

**How they're specified**: A radiator is sized for the heat output required for the room (in BTU or kW). For a well-insulated 4m × 5m single-storey rear extension (20m²) in London, a 2,500–3,500 BTU radiator is typically sufficient. Your plumber or heating engineer should calculate the required output based on the room's heat loss.

• **Placement in open-plan extensions**:
• The challenge with radiators in open-plan extensions is finding a position that:
• •Does not conflict with kitchen units
• •Is not on the glazed rear wall (the standard under-window position is not possible with bifold or sliding doors)
• •Does not restrict furniture arrangement

Options include: a tall vertical radiator on a side wall; a perimeter radiator on the kitchen side; or a short low radiator beneath the window of the side return.

• **Radiator costs for extension heating (supply and install, London 2025)**:
• •Single standard radiator (extension connection from existing system): £400–£900
• •Towel rail (bathroom extension): £300–£600
• •Full pipework run from boiler to extension radiator: £600–£1,500 additional (depending on distance)

• An air source heat pump (ASHP) works by extracting heat from outside air and delivering it to the heating system — generating 3–4 kWh of heat for every 1 kWh of electricity consumed. For extensions, ASHPs work particularly well when combined with UFH because:
• •UFH operates at low water temperatures (35–45°C) which is precisely where ASHPs are most efficient
• •A well-insulated extension requires only a low-temperature heat input — perfect for ASHP

• **Cost of an ASHP system**:
• •Supply and installation: £8,000–£15,000 for a residential heat pump
• •Less the Boiler Upgrade Scheme grant (England): £7,500 government grant — reducing the net cost significantly
• •Running costs depend on electricity tariff and the ASHP's COP (coefficient of performance) — typically 3–4× more efficient than a gas boiler in heating terms, but electricity is more expensive per unit than gas (approximately 3–5× in 2025)

**For extensions specifically**: Rarely is a heat pump installed for an extension alone — the investment is too large for a single room. The decision to install an ASHP is typically whole-house or is made when a boiler is due for replacement.

**Electric UFH (dry electric)**: An alternative for extensions where adding wet UFH to the central heating system is impractical: electric UFH mats or cables embedded under tiles. Lower installation cost (no manifold, no screed — just the mat and a thermostat). Higher running cost than wet UFH from a gas boiler (electricity vs gas). Suitable for small extensions or bathrooms.

Electric UFH cost for a 25m² extension: £800–£2,000 (mat + thermostat + installation).