Heating a London Extension: Radiators, Underfloor Heating, and Boiler Capacity

**The existing boiler capacity question**:

• Before specifying the heating for a London extension, the first question is whether the existing boiler has enough remaining capacity to heat the additional space. A typical London Victorian terrace heating system is sized for the original house — it may have little or no spare capacity for an extension, particularly if:
• •The existing boiler is more than 10–15 years old (older boilers lose efficiency over time and may already be running near their rated output)
• •The extension is larger than approximately 15–20m² (a small extension may be manageable on the existing system; a larger open-plan kitchen-diner extension almost certainly requires a capacity check)
• •The existing system was under-sized even before the extension (cold radiators, slow heat-up times, insufficient hot water)

**How to check existing boiler capacity**:

1. Locate the existing boiler manufacturer plate (kW output rating) — typically on the front or top of the boiler 2. Commission a heat loss calculation for the entire property (including the new extension) from a Gas Safe registered heating engineer — this calculates the total heat demand of the property and compares it to the boiler's rated output 3. If the boiler's rated output (in kW) is less than the total calculated heat demand (in kW), the boiler must be upgraded or supplemented

**Typical heat loss figures for London extension spaces (2025, well-insulated to Part L)**:

• As a rough guide (an accurate heat loss calculation by an engineer is essential for design):
• •25m² single-storey rear extension (Part L insulation; double glazed): approximately 1.5–2.5 kW heat loss at the design temperature (-3°C external; 20°C internal)
• •Open-plan kitchen-diner-living (40m² combined, including existing opened-up house): approximately 3.0–5.0 kW additional heat demand

**Boiler upgrade thresholds for London extension projects**:

• •If the extension adds more than approximately 2.0–2.5 kW to the system heat demand, and the existing boiler is operating close to its rated output, a new boiler is likely required
• •Modern combi boilers for London Victorian terraces: typically 24kW, 30kW, or 35kW rated output (the 30kW or 35kW size is appropriate for most 3–4 bedroom London terraces with a rear extension)
• •Cost of a new combi boiler supply and fit in London (2025): approximately £1,500–£3,000 for a standard Worcester Bosch, Viessmann, or Vaillant combi; higher for system boilers with cylinder

**Radiators vs. underfloor heating — the key decision for a London extension**:

• For a London extension, the heating method choice is between:
• •Extending the existing radiator system (adding radiators in the extension connected to the existing pipework and boiler)
• •Installing wet underfloor heating (UFH) in the extension slab or screed, connected to the existing boiler system
• •Installing electric underfloor heating (direct electric UFH — a mat below the tiles; no connection to boiler required)

• This decision is affected by:
• •The floor construction (underfloor heating in a concrete slab or screed is the easiest route; UFH in a timber beam-and-block floor requires specific joisted UFH panels)
• •The kitchen layout (kitchen units with a base plinth make UFH the preferred solution — radiators behind kitchen units are inaccessible and thermally poor)
• •The existing boiler temperature (older high-temperature boilers are inefficient when paired with wet UFH which requires 35–45°C flow temperature vs. radiators at 70–80°C flow temperature)
• •The desired room aesthetic (no radiators on walls — preferred by many London homeowners for an open-plan kitchen extension)
• •Cost (UFH has a higher installation cost than radiators but no ongoing maintenance cost differential)

**Radiator sizing for a London extension**:

Radiators must be sized to provide sufficient heat output at the design water flow temperature to meet the room heat demand. The relationship:

Heat output (W) = Radiator output rating at standard test conditions (75°C flow / 65°C return / 20°C room) × ΔT correction factor

• For an older high-temperature London heating system (flow temperature 75–80°C):
• •A standard double-panel convector radiator (600×1200mm Type 22) has a rated output of approximately 2,500–3,000W at ΔT50 (75/65/20°C)
• •This is usually sufficient for a well-insulated 15–20m² extension space at high flow temperature

• For a condensing boiler optimised for lower flow temperatures (60°C):
• •The ΔT correction factor is approximately 0.75 — so the same radiator outputs approximately 1,875–2,250W
• •Larger radiators, or additional radiators, are required to compensate for the lower output at reduced temperature

**Common radiator positions in London extension spaces**:

• •Under windows: the traditional and thermally correct position — the rising convective airflow from the radiator counteracts the downdraft cold air from the window, creating a comfortable thermal curtain. In a contemporary extension with large bifold doors, a radiator below or adjacent to the bifold is the thermally logical position
• •Behind kitchen units: avoid — radiators behind kitchen base units (even with a plinth gap) are thermally poor and almost inaccessible for maintenance and bleeding. In an open-plan kitchen extension, UFH is the preferred solution for the kitchen zone; a radiator in the living/dining zone is acceptable
• •Perimeter of the extension: in a large open-plan space, single radiators on perimeter walls (where not occupied by glazing or kitchen units) provide an even heat distribution

**Pipework for extending the radiator system into the extension**:

• Extending the radiator system from the existing house into the extension requires:
• •Running new 22mm or 15mm copper or plastic MLCP (multi-layer composite pipe) feed and return pipes from the existing manifold or ring main to the new radiators
• •The extension is typically at the end of the heating circuit — the pipe run from the last existing radiator to the new extension radiator(s) should be calculated for heat loss in the pipe run and pressure drop
• •Thermostatic radiator valves (TRVs) on all extension radiators — allows individual room temperature control
• •Isolation valves on each radiator — for maintenance and bleeding
• •The new pipework must be flushed and inhibitor solution added to the system when commissioning

**Pipework routing in a London extension**:

• Pipes can be routed:
• •Under the concrete floor slab in conduit (must be installed at first-fix stage, before the slab is poured — cannot be added later without core drilling)
• •Through the wall from the existing house (typically through the rear wall of the original house, into the ceiling/void of the extension, and down to the extension floor level)
• •Surface-run (cased in boxing) where a concealed route is not practical

For a wet underfloor heating system where the manifold is in the extension, a single 22mm flow and return from the boiler to the UFH manifold simplifies the pipework considerably.

**Part L and low-temperature heating systems for extensions**:

Building Regulations Approved Document L1B (Conservation of fuel and power in existing dwellings) applies to heating systems installed in extensions. The SAP (Standard Assessment Procedure) energy calculation used to demonstrate extension compliance can be affected by the heating system specification — a lower flow temperature heating system (UFH or oversized radiators at lower temperature) improves the SAP score because condensing boiler efficiency is higher at lower return temperatures.

• For a London extension connected to an existing gas boiler:
• •Ensure the boiler is a high-efficiency condensing combi or system boiler (minimum ErP class A rated — all boilers sold in the UK since 2018 must be ErP A rated)
• •Ensure the boiler is correctly commissioned for the system including the extension — flow temperature set to the minimum compatible with the heat emitters (UFH: 35–45°C; large radiators: 55–60°C where heat output allows)
• •TRVs on all radiators + programmer/room thermostat (or smart thermostat such as Hive, Nest, or Tado): required for Part L compliance in new extensions in England

**Air source heat pump (ASHP) integration with a London extension — considerations**:

Many London homeowners enquire about installing an air source heat pump when carrying out a significant extension or refurbishment. ASHPs are efficient heating sources (Coefficient of Performance 2.5–4.0 in London conditions), reduce gas consumption, and may qualify for the Boiler Upgrade Scheme grant (£7,500 in 2025).

However, ASHPs are significantly more complex to integrate with an existing London Victorian terrace heating system than a straightforward boiler replacement, because:

*1. System design temperature*: ASHPs operate at much lower flow temperatures (35–50°C) than gas boilers (65–80°C). An existing radiator system designed for 70–80°C flow temperature will be severely undersized for ASHP operation — every radiator in the property must be assessed and likely upsized (or replaced) to provide adequate heat output at the lower ASHP flow temperature. This is a significant cost item in a whole-house ASHP retrofit — typically £2,000–£8,000 in London depending on the number of radiators.

*2. Hot water cylinder*: A combi boiler heats hot water on demand. An ASHP requires a hot water cylinder (typically 200–300 litres for a London family home) — the cylinder stores ASHP-heated water at approximately 55°C for domestic hot water use. Adding a cylinder requires significant space (typically 0.5–0.8m² floor area; 1.8m height).

*3. Noise*: ASHP units are installed externally (garden, side return, flat roof — wherever Permitted Development or planning allows) and generate noise (approximately 40–45 dB at 1m — roughly the sound of a quiet conversation). In London terraced houses with close neighbouring properties, this can be a constraint.

*4. An extension is the ideal time to design for ASHP*: The extension provides an opportunity to install UFH in the slab (the ideal heat emitter for ASHP — low flow temperature; maximum heat exchange area) and to upgrade the insulation of the existing house as part of the project. A well-insulated property with UFH is the best platform for ASHP operation. If ASHP is a future intent, specify the extension heating as UFH now.

**Costs for heating a typical London rear extension (2025)**:

| Heating option | Supply + installation cost (typical 25m² London extension) | |---|---| | Add 2× radiators to existing system (new pipework from existing circuit) | £600–£1,200 | | Wet UFH in concrete slab (manifold + pipework + commissioning; excludes screed) | £1,200–£2,500 | | Electric UFH mat under tiles (no boiler connection) | £400–£900 | | New 30kW combi boiler (supply + fit; replacement only) | £1,800–£3,200 | | ASHP with new cylinder and radiator upgrades (whole-house package) | £8,000–£18,000 (before BUS grant) |