Heating Systems for House Extensions and Renovations: What to Specify in London

**The first question: can the existing boiler cope?**

Before specifying heating for the extension, establish whether the existing boiler has sufficient output capacity to heat the additional floor area without replacement.

• *How to assess boiler capacity*:
• •Identify the existing boiler output (kW) — on the boiler data plate or manual
• •Calculate the heat loss from the extension (W/m² of floor area × the floor area) — a well-insulated single-storey rear extension in London typically has a heat loss of 30–50 W/m²; a less well-insulated extension may be 60–80 W/m²
• •For a 25m² extension at 50 W/m² heat loss = 1,250W = 1.25 kW of additional demand
• •Compare the additional demand with the boiler's spare capacity (installed output minus current system demand)

*In practice*: Most modern combi boilers (24kW–30kW) have sufficient spare capacity to heat a standard single-storey rear extension (20–35m²) without replacement. The critical check is whether there is still spare capacity after accounting for the existing rooms.

• *When boiler replacement is warranted*:
• •The existing boiler is over 15 years old (efficiency below 80–85% compared with 90–94%+ for modern condensing boilers)
• •The boiler output is inadequate for the additional demand even with calculation
• •The existing boiler requires frequent repairs or shows signs of imminent failure — better to replace proactively during the building works than reactively when the house is occupied
• •The project involves a significant increase in hot water demand (additional bathrooms, larger household) that the existing combi cannot supply

**Boiler sizing for a renovated or extended house**:

• A correctly sized condensing gas combi boiler for a typical extended 3-bed Victorian London terrace:
• •House pre-extension (120m² GIA, well-draught-proofed but limited insulation): typically 24kW–28kW demand
• •After single-storey rear extension (adding 25m²): total approximately 25–27 kW — a 28kW or 30kW combi is typically adequate
• •After double-storey extension (adding 50m²): total approximately 27–32 kW — a 35kW combi or system boiler with unvented cylinder may be more appropriate

*Note*: These are illustrative figures; a proper heat loss calculation is required for accurate boiler sizing. An oversized boiler cycles on and off excessively (short-cycling), reducing efficiency and increasing wear.

**System boiler vs. combi — when the renovation is the time to switch**:

*Combi boiler*: Heats water on demand — no hot water cylinder required. Adequate for 1–2 bathroom households with modest hot water demand. Standard in most London Victorian terraces.

*System boiler + unvented (pressurised) cylinder*: The system boiler heats the heating circuit and charges a hot water cylinder. Better for 3+ bathroom households, or where simultaneous demand from multiple outlets (two showers running simultaneously) is a concern. Requires a hot water cylinder (typically 150–300 litres), taking up approximately 0.5–0.8m² of floor area.

If the renovation involves adding a second or third bathroom, or if the household will have high simultaneous hot water demand, switching from a combi to a system boiler with unvented cylinder during the renovation is the most cost-effective time to do so — pipework is open, cylinder location can be considered in the design.

**Part L and the Boiler Plus requirements**:

• All new or replacement boilers in England must meet the Boiler Plus requirements (in force since April 2018):
• •Minimum ErP (Energy-related Products) efficiency rating of 92%
• •One of the following additional energy-saving measures: time and temperature zone control, a flue gas heat recovery system, a weather compensator, or a load compensator

In practice, all new modern condensing boilers meet the efficiency requirement; the zone control, weather compensator, or load compensator requirement influences the controls specification.

**Why extensions are the ideal opportunity for UFH**:

• Underfloor heating (UFH) in a house extension is significantly easier to install than retrofitting UFH into an existing house because:
• •The structural floor of the extension is typically a new concrete slab — UFH pipes or mats can be laid into the screed above the slab before the floor finish is installed
• •There are no existing floor finishes to lift and relay
• •The extension heating circuit can be added to the existing boiler system relatively easily via a manifold

• *When UFH is particularly suitable for extensions*:
• •Large open-plan kitchen-dining-living extensions — underfloor heating eliminates the need for radiators on the walls, leaving the walls free for furniture, kitchen units, and glazed doors
• •Extensions with large areas of bifold doors — large glazed areas lose heat; UFH at the perimeter of the glazing heats the floor surface closest to the cold glass, reducing the sensation of draught and improving comfort
• •Extensions with polished concrete, stone, or large-format tile floor finishes — these hard floors conduct heat well from UFH; UFH performs poorly under thick carpet

**Wet UFH vs. electric UFH for extensions**:

• *Wet UFH (water-based, connected to the boiler)*:
• •Pipes embedded in the floor screed carry hot water from the boiler (typically 35–45°C flow temperature for UFH vs. 70–80°C for conventional radiators)
• •Significant efficiency advantage when combined with a condensing boiler (the boiler runs in condensing mode more effectively at lower flow temperatures) and especially with a heat pump
• •Capital cost: approximately £50–£80/m² installed for wet UFH in a new concrete slab extension
• •Slow to respond (heat-up time 30–60 minutes) — better suited to 'set and forget' constant heating than rapid response heating

• *Electric UFH (resistance heating mat or cable)*:
• •Thin cable or mat below the floor finish — not embedded in screed; can be installed above an existing floor
• •Very fast response (10–20 minutes)
• •Running cost significantly higher than wet UFH using gas or heat pump (electricity is approximately 3.5× the cost per kWh of gas)
• •Appropriate where: the extension is occasionally heated (utility room, bathroom used infrequently); the extension is very small (single bathroom); wet UFH connection to the boiler is impractical
• •Capital cost: approximately £20–£40/m² installed

**Screed specification for wet UFH**:

Wet UFH pipes require a screed overlay of minimum 65mm above the pipes (standard sand/cement screed) or minimum 35mm (liquid anhydrite screed). The screed depth adds to the overall floor build-up and affects the relationship between the extension floor level and the existing house floor level.

• *Floor finish compatibility with wet UFH*:
• •Porcelain or ceramic tiles: ✓ Excellent conductor — highly compatible
• •Stone (limestone, slate, marble): ✓ Excellent
• •Polished concrete: ✓ Excellent
• •Engineered wood (18mm): ✓ Compatible — use boards rated for UFH use
• •Solid wood (22mm+): Limited — thicker solid wood floors insulate and slow response time
• •Vinyl / LVT: ✓ Compatible (check manufacturer's maximum UFH temperature)
• •Carpet: Not recommended — carpet over UFH insulates the floor and prevents efficient heat transfer

**Heat pumps and why extension heating specification should consider them**:

The UK Government's Clean Heat Market Mechanism and the phasing out of new gas boiler installations (currently targeted from 2035) means that new extensions being designed and built now may still be in use when gas boilers are being replaced with heat pumps.

• A heat pump (air source or ground source) heats water to a lower flow temperature than a gas boiler — typically 35–50°C (air source) compared with 70–80°C for a boiler. At lower flow temperatures:
• •Conventional radiators underperform (they are designed for 70°C+ flow)
• •Underfloor heating (designed for 35–45°C flow) performs excellently

This is why underfloor heating in the extension significantly improves the property's suitability for a future heat pump retrofit — when the gas boiler is replaced by a heat pump, the UFH in the extension will work well at heat pump temperatures; only the existing radiators in the rest of the house may need upsizing.

**What 'heat pump ready' means for an extension**:

• •*High-quality fabric insulation*: Heat pumps are most efficient in a well-insulated building (low heat loss) — specify the extension insulation to Part L 2021 standards or better (U-values: external walls ≤ 0.18 W/m²K; ground floor ≤ 0.13 W/m²K; roof ≤ 0.11 W/m²K)
• •*Underfloor heating in the extension ground floor*: Designed for 35–45°C flow temperature, compatible with heat pump output
• •*Pipework sizing*: Size the pipework for lower temperature operation — slightly larger bore pipework reduces the flow rate required to achieve the same heat output
• •*No combi boiler*: Heat pumps charge a hot water cylinder (they cannot operate as instantaneous water heaters) — if the extension project involves a boiler replacement, a system boiler with cylinder is more heat-pump-ready than a combi

**The current (2025) picture for heat pumps in London**:

• Air source heat pumps in London face specific challenges:
• •Most London houses are solid-wall construction — heat pumps are most efficient in well-insulated buildings; a Victorian solid-wall house with limited insulation has a high heat loss that reduces heat pump efficiency
• •Space for the external unit: the compressor unit (approximately 0.9m × 0.3m × 0.8m) must be positioned outside the building, typically in the garden or on the side of the house, with clear air flow on all sides
• •Permitted development for heat pump installation: ASHP (air source heat pump) installation is permitted development in England provided certain conditions are met (volume under 0.6m³, not on a wall or roof facing a highway in a Conservation Area, not within 1m of a property boundary, the property has not previously had a heat pump installed under PD)
• •The Boiler Upgrade Scheme (BUS) provides a £7,500 grant toward the cost of an air source heat pump installation

*Practical advice*: For an extension project in 2025, specifying UFH in the extension and ensuring the extension has high-quality insulation are the most cost-effective heat-pump-ready measures. A full heat pump installation typically costs £10,000–£18,000 (before BUS grant) and is a separate decision from the extension project.