Heating a London Extension: Options, Costs, and What Building Regulations Require

**What Part L 2022 requires for a new extension:** Part L of the Building Regulations (Conservation of Fuel and Power) sets minimum thermal performance requirements for building fabric. For a new extension, the requirements are more demanding than for the existing house. The key limiting U-values for an extension under Part L 2022 are:

• •External walls: 0.26 W/m²K
• •Roof (flat or pitched at ceiling level): 0.16 W/m²K
• •Ground floor: 0.18 W/m²K
• •Windows and glazed doors: 1.6 W/m²K

These are the worst-case limiting values — the extension fabric must achieve at least this level of thermal performance. In practice, a well-designed extension will exceed these values.

**Why this matters for heating design:** The better the fabric performance (insulation, glazing specification, thermal bridging control), the lower the heat demand of the extension and the more manageable the load on the existing heating system. An extension specified to better than minimum Part L performance may need relatively little additional heating capacity — particularly with south-facing glazing (solar gain) and good airtightness.

Conversely, a large rear extension with a significant area of glass (bifold or sliding doors, rooflight, glazed roof) will have a higher heat demand even if the opaque elements meet Part L — glass, even at 1.6 W/m²K, loses heat far faster than an insulated wall.

**Building Regulations application:** The extension must be included in a Part L calculation (SAP or simplified calculation) submitted as part of the Building Regulations application. The approved inspector or local authority Building Control will check that the notional dwelling calculations are met. This is submitted as part of the Full Plans application or Building Notice.

**Boiler capacity check — the first step:** The most common approach for heating a London extension is to extend the existing wet central heating system (radiators or underfloor heating connected to the existing combi boiler or system boiler). Before assuming this is possible, a heat loss calculation must establish:

1. The additional heat demand of the new extension space (in kilowatts) 2. The remaining spare capacity of the existing boiler

Most combination boilers installed in London houses are sized for the existing dwelling — many are running close to their maximum output capacity. Adding the heat demand of a new extension (typically 1–4 kW for a well-insulated single-storey rear extension) may push the boiler over its capacity, resulting in: underheating of the extension on cold days; underheating of the rest of the house as the boiler struggles; or boiler lockout.

• **What to do if the boiler is at capacity:**
• •Boiler replacement with a higher-output model (e.g. upgrading from a 24 kW to a 28 kW or 32 kW combi): typically £1,200–£2,500 installed, gas engineer required
• •Addition of a supplementary heat source in the extension only (electric UFH, electric panel heater, or air-source heat pump) — leaving the existing boiler serving the rest of the house
• •Addition of an ASHP (Air Source Heat Pump) to serve the extension as a separate heating zone

**Extending the radiator circuit:** If the boiler has capacity, extending the radiator circuit to the extension is straightforward — the plumber runs new 22mm flow and return pipe from the existing manifold or the nearest convenient connection point to new radiators in the extension. The extension radiators must be sized to the heat loss of the new space.

For an open-plan kitchen-dining extension, a single oversized radiator (or two standard radiators) is typically sufficient. The radiator location should be designed with the room layout in mind — a radiator positioned under a window (traditional) or on a side wall where furniture arrangement is unlikely to obstruct it.

**Underfloor heating as the primary emitter:** For a new extension, water underfloor heating (UFH) connected to the existing boiler circuit is increasingly popular — particularly where the extension has a poured concrete ground-bearing slab (typical for single-storey rear extensions). Water UFH:

• •Requires a minimum screed depth of approximately 65mm from the top of the pipe to the finished floor surface (for a sand-cement screed or liquid screed)
• •Requires a mixing valve (manifold with actuators) to reduce the boiler flow temperature from 60–70°C to the UFH design temperature (typically 40–50°C for heating, 35–45°C with heat pump)
• •Provides even, low-level warmth — compatible with stone tiles, porcelain, LVT, and engineered wood (within temperature limits)
• •Takes longer to respond to temperature changes than radiators — UFH is a background heating system, not a quick-response system; it is best left on a schedule rather than turned on and off
• •Needs to be designed and commissioned by a specialist — the pipe layout (serpentine or spiral), pipe centres, and manifold design all affect performance

**Air Source Heat Pumps (ASHP) for extensions:** An air source heat pump extracts heat from outside air and delivers it as low-temperature heat to a wet underfloor heating or low-temperature radiator system at a Coefficient of Performance (CoP) typically 2.5–4.0 — meaning for every 1 kW of electricity consumed, 2.5–4.0 kW of heat is delivered.

ASHP considerations for a London extension:

• •**The extension is an ideal ASHP application**: new build / new extension zones allow the fabric, emitter system, and ASHP to be designed together from the outset — maximising efficiency
• •**UFH is the natural emitter for ASHP**: ASHP operates most efficiently at lower flow temperatures (35–50°C), and water UFH is designed for exactly this range
• •**Planning for the external unit**: the outdoor unit (typically the size of a large suitcase, installed on an external wall or on a ground slab) must be positioned with adequate airflow clearance, away from bedrooms (noise at night), and in most London residential cases within permitted development (PD) for air source heat pumps — but NOT on a listed building without consent, and NOT on the principal elevation facing a highway
• •**Boiler Zone Control approach**: the ASHP serves the extension (via UFH) while the existing gas boiler continues to serve the rest of the house — each zone on its own controls. A cost-effective approach when the boiler is at capacity and the extension is a significant space
• •**Costs**: domestic ASHP for extension application (supply, install, commissioning) £4,000–£9,000. Boiler Upgrade Scheme (BUS) government grant of £7,500 available for full ASHP replacement of gas heating — this grant applies to whole-house replacement, not extension-only top-up systems, so confirm eligibility with the installer

**Electric underfloor heating (mat UFH):** For a bathroom, en-suite, or a smaller extension zone where water UFH is impractical, electric mat underfloor heating is the simplest solution. Electric mat UFH:

• •Is a resistance heating element — not an efficient primary heating source for large spaces (100% efficiency = expensive to run at scale)
• •Is cheap to install (£30–£80/m² installed including thermostat and wiring)
• •Is ideal for bathrooms and wet rooms where the floor area is small and the heat is a comfort supplement rather than the primary space heating source
• •Requires a dedicated circuit from the consumer unit (Part P — electrical certification required)
• •Works under porcelain tile, natural stone, and LVT (check manufacturer specification for the specific product)

**Zone control:** A new extension should be on its own heating zone — separate thermostat and programmer from the rest of the house. Without zone control, the extension and the existing house will heat and cool as one system, making it impossible to maintain comfortable temperatures in the extension independently. Zone control valves (motorised zone valves) connected to a room thermostat in the extension are a standard addition when extending the radiator circuit.