Energy Efficiency in London Extensions: Part L, Insulation, and Smart Design

Part L of the Building Regulations (Conservation of Fuel and Power) sets the minimum energy performance standards for new extensions and renovations in England. The relevant documents are Approved Document L1B (Conservation of Fuel and Power in Existing Dwellings). The key Part L requirements for a London home extension in 2025: U-values: U-value is the rate of heat transfer through a building element (lower is better — lower U-value = less heat loss). The minimum (maximum permitted) U-values for a new extension under Part L1B are: External walls: 0.28 W/m²K; Roofs (flat or pitched): 0.16 W/m²K (for a flat extension roof — the most common type in London); Ground floor: 0.22 W/m²K; Windows and external doors: 1.4 W/m²K; Rooflights: 1.6 W/m²K. These are minimum standards — not aspirational targets. Most responsible extension contractors and architects specify significantly better U-values than the Part L minimum — particularly for roofs and walls, where additional insulation is low cost relative to the benefit. Target specifications for a high-performance London extension: External walls: 0.18 W/m²K (100mm full-fill cavity or 120mm PIR board external insulation); Flat roof: 0.15 W/m²K (130-150mm PIR board above structural deck in a warm roof build-up); Ground floor: 0.15 W/m²K (100mm PIR below ground-bearing slab); Windows: 1.2 W/m²K (thermally broken aluminium, double-glazed low-E). Improvement to the existing dwelling: where the extension links to the existing dwelling through a structural opening (removing the existing rear wall for an open-plan kitchen-dining), Part L1B also requires the homeowner to consider making proportionate energy improvements to the existing property — including insulating any accessible roof spaces, replacing inefficient boilers if due for replacement, and ensuring any disturbed elements are upgraded to Part L standards. The energy compliance calculation (SAP or simplified Part L compliance check) must be carried out by an energy assessor or by the architect/contractor if they have Part L expertise. A Certificate of Compliance with Part L must be available for inspection on Building Control sign-off.

Insulation is the most cost-effective energy investment in a London extension — a well-insulated extension retains heat in winter and stays cooler in summer. The key construction elements to insulate correctly: Flat roof insulation (most critical): a flat-roofed London extension with a warm roof build-up (all insulation above the structural deck) should use rigid PIR (polyisocyanurate) insulation boards — Celotex, Kingspan, or equivalent — to achieve a U-value of 0.15 W/m²K or better. Insulation thickness required: 100mm PIR achieves approximately 0.18-0.20 W/m²K; 130mm PIR achieves 0.15 W/m²K; 150mm PIR achieves 0.12 W/m²K. Tapered insulation (boards that are thicker on one side to create the required fall to the drain outlet) can achieve both the required thermal performance and the required drainage gradient — a dual-purpose specification. External cavity wall insulation: a new-build cavity wall extension in London should use full-fill rigid PIR cavity boards (Celotex Cavity Board, Kingspan Kooltherm K108) to fill the entire cavity — typically 75mm or 100mm full-fill PIR in a 100mm cavity. Full-fill PIR cavity achieves approximately 0.20-0.22 W/m²K. An alternative is partial fill cavity with a wider cavity (150mm cavity with 90mm mineral wool partial fill and 60mm air gap) — achieves similar U-values but uses a more traditional mineral wool specification. Do not use blown mineral wool (retrofitted full fill) in new-build construction — this is a retrofit system for existing cavity walls, not for new-build. Ground floor insulation: a ground-bearing concrete slab with 100mm PIR below (Celotex FB5, Kingspan TF70, or equivalent) achieves a ground floor U-value of approximately 0.15-0.18 W/m²K. The insulation is placed on a 1200-gauge DPM (damp-proof membrane) on the hardcore blinding, before the concrete slab is poured. For a suspended timber floor (common in London Victorian extension additions), 100mm mineral wool between the joists provides approximately 0.20-0.25 W/m²K — less insulated than a ground-bearing slab with rigid PIR. Thermal bridging: even with well-insulated wall, roof, and floor elements, heat can bypass the insulation through thermal bridges — junctions where the insulation layer is interrupted by a structural element (a joist end that penetrates the insulation layer; a steel beam that bridges the inner and outer wall leaf). Thermal bridging at junctions (windows, roof-wall junction, wall-floor junction) is assessed as part of the Part L compliance calculation. Minimising thermal bridging at these junctions — by detailing insulation continuity at all junctions — is an important element of a high-performance London extension.

Air source heat pumps (ASHPs) are an increasingly common specification in London home extensions — driven by the zero-rated VAT on heat pump installation (since February 2024), the government's Boiler Upgrade Scheme (a £7,500 grant for ASHP installation replacing a gas boiler), and the rising availability of installers. How a heat pump works in a London extension context: an ASHP extracts heat from the outside air and transfers it, via refrigerant cycle, to the central heating system. ASHPs work most efficiently at lower flow temperatures (35-45°C) than a standard combi boiler system (60-80°C) — this means that the heating emitters (radiators or underfloor heating) in the extension must be sized for low-temperature operation. Underfloor heating (UFH) is ideally suited to heat pump operation — UFH typically operates at 30-40°C flow temperature, which is ideal for ASHP efficiency. For a London extension with UFH and an ASHP, the combination provides a highly efficient, low-carbon heating system. Standard radiators sized for 80°C flow temperature will be inadequately sized for an ASHP at 45°C — they will need to be oversized (larger surface area) or replaced with low-temperature panel radiators to provide the same heat output at the lower flow temperature. Space for the ASHP unit: an air source heat pump unit (the outdoor condensing unit, similar in appearance to an air conditioning compressor) must be positioned outside — in the garden, on the side of the house, or on a roof (if structural loading and acoustic impact are acceptable). For London terrace houses with small gardens, the garden is the most common location. Planning permission is not usually required for a standard ASHP installation in a rear garden — most ASHPs are permitted development. When a heat pump makes the best sense for a London extension: when the extension includes UFH (making the most of the ASHP's low-temperature efficiency); when the homeowner is replacing an aging gas boiler and the extension is triggering an M&E review of the whole house; when the property is well-insulated (a poorly insulated existing house means the ASHP runs at higher flow temperatures to compensate, reducing efficiency). When a heat pump is less suitable: where the existing house heating system uses standard-sized radiators designed for 80°C flow (replacing these across the whole house is a significant additional cost); where the garden is very small and the outdoor unit would cause noise nuisance to close neighbours.

For London homeowners seeking maximum energy efficiency — well beyond Part L minimum standards — airtightness and Mechanical Ventilation with Heat Recovery (MVHR) are the two most impactful additional measures. Airtightness: uncontrolled air infiltration (draughts through gaps in the building fabric — around windows, at floor-wall junctions, through socket outlets, around service penetrations) accounts for 20-40% of heat loss in a typical London home. A standard London extension built to normal construction standards has an air permeability of approximately 5-10 m³/h/m² at 50Pa (the standard test pressure). A Passivhaus-standard extension achieves an air permeability of 0.6 m³/h/m² or less — reducing uncontrolled heat loss by 80-90%. Achieving low airtightness requires: a continuous airtightness layer (membrane or masonry coat) on the warm side of the insulation; meticulous sealing of all penetrations (pipes, cables, structural connections) through the airtightness layer; attention to junctions at window reveals, roof-wall junctions, and floor-wall junctions. An airtightness test (the blower door test, carried out after the building is sealed but before finishes are applied) measures the achieved air permeability and confirms whether the target has been met. For a standard London extension, an airtightness target of 3-5 m³/h/m² (between Part L minimum and Passivhaus) is achievable without specialist Passivhaus detailing — and significantly improves comfort and energy performance. MVHR (Mechanical Ventilation with Heat Recovery): in a highly airtight building, deliberate controlled ventilation is required to maintain air quality (CO², humidity, and VOC levels). An MVHR unit extracts stale warm air from the kitchen, bathroom, and utility spaces, passes it through a heat exchanger where 80-90% of the heat is recovered from the outgoing airstream, and supplies pre-warmed fresh air back into the bedrooms and living spaces. The result: the building is continuously ventilated with fresh air, with minimal heat loss. MVHR is most valuable in a highly airtight new extension — in a leaky existing house, it is less effective because uncontrolled infiltration bypasses the MVHR. MVHR unit cost and installation: supply of MVHR unit (Paul Novus, Zehnder ComfoAir, Brink Flair): £1,500-£4,000 supply; ductwork and installation: £2,000-£5,000. Total MVHR supply and install for a London extension: £3,500-£9,000.