Building Regulations Part L for London Extensions: Energy Efficiency Requirements Explained

**What is a U-value?**

• A U-value (thermal transmittance) measures how much heat passes through a building element per unit area per degree of temperature difference between inside and outside. It is measured in W/m²K (watts per square metre per kelvin of temperature difference). The lower the U-value, the better the insulation performance:
• •A well-insulated wall: U = 0.18–0.25 W/m²K
• •A double-glazed window: U = 1.2–2.0 W/m²K (whole window, not just glass)
• •A poorly insulated older extension wall: U = 1.0–1.5 W/m²K

**Approved Document L2B (ADL2B — works to existing dwellings) — the relevant document for London extensions**:

For extensions and loft conversions to existing dwellings (the most common situation for London homeowners), the relevant part of the Building Regulations is Approved Document L, Volume 1 (for dwellings), Section 4 (Consequential improvements and U-value standards for extensions). The key document was the 2021 update to ADL (effective from June 2022), which tightened U-value requirements significantly compared to the previous 2013 standard.

**The backstop U-values — the minimum mandatory standard for London extensions (2022 ADL)**:

The 'backstop' values are the worst-performing values that any element in a new extension may have. They are the minimum acceptable standard under Part L — an extension built to exactly the backstop values is compliant, but not highly efficient:

| Building element | Backstop U-value (maximum allowed) | |---|---| | External wall | 0.26 W/m²K | | Flat roof | 0.15 W/m²K | | Pitched roof (insulation between rafters or at ceiling level) | 0.15 W/m²K | | Ground floor (solid slab) | 0.25 W/m²K | | Ground floor (beam and block with void) | 0.25 W/m²K | | Windows (whole window Uw) | 1.60 W/m²K | | Rooflights | 1.80 W/m²K | | Doors (external, part-glazed) | 1.60 W/m²K | | Bi-fold / sliding doors (whole unit Uw) | 1.60 W/m²K |

**What insulation is needed to achieve the backstop U-values?**

• The exact insulation thickness needed depends on the construction type and the insulation product's thermal conductivity (λ value, measured in W/mK). Common insulation products and their typical thermal conductivities:
• •PIR (polyisocyanurate) rigid board (e.g., Kingspan K-series, Recticel Poweriso): λ = 0.022–0.026 W/mK — the best-performing common board insulation
• •EPS (expanded polystyrene): λ = 0.033–0.038 W/mK
• •Mineral wool (semi-rigid or flexible batts): λ = 0.032–0.044 W/mK
• •Aerogel insulation: λ = 0.015 W/mK (extremely thin, very expensive — used where space for insulation is severely constrained)

*Indicative insulation thicknesses to achieve the 0.26 W/m²K wall backstop*:

• For a typical London cavity wall extension (100mm outer brick, 100mm cavity, 100mm inner block, 13mm plaster):
• •If the full 100mm cavity is filled with full-fill mineral wool: U ≈ 0.26 W/m²K (exactly at the backstop)
• •If partial fill (75mm PIR board, 25mm residual cavity): U ≈ 0.22 W/m²K (better than backstop)
• •If 100mm PIR board in the cavity (full fill): U ≈ 0.18 W/m²K (significantly better than backstop)

*Indicative insulation for the 0.15 W/m²K flat roof backstop*:

• For a warm flat roof (insulation above the structural deck, below the waterproofing membrane):
• •100mm PIR board (λ = 0.024): U ≈ 0.22 W/m²K — NOT compliant with the 0.15 backstop
• •150mm PIR board: U ≈ 0.15 W/m²K — just at the backstop
• •160–170mm PIR board: U ≈ 0.14–0.13 W/m²K — better than backstop

This is a common area where extensions fail Part L — 100mm flat roof insulation was adequate under the pre-2022 standard but is no longer sufficient. 150mm PIR minimum is the practical minimum for a compliant post-2022 flat roof extension.

*Indicative insulation for the 0.25 W/m²K floor backstop*:

• For a solid ground-bearing slab (125mm concrete slab, insulation below):
• •75mm PIR (λ = 0.022): U ≈ 0.28 W/m²K — NOT compliant
• •100mm PIR: U ≈ 0.22 W/m²K — compliant
• •125mm PIR: U ≈ 0.18 W/m²K — better than backstop

Again: 75mm floor insulation was adequate under the 2013 standard but is no longer sufficient. 100mm minimum PIR below the slab is the practical minimum for Part L 2022 compliance in a typical London extension.

**Thermal bridging — the junction details that can undermine good insulation**:

A thermal bridge is a point in the building fabric where heat flows more easily than through the surrounding insulation — typically at junctions between elements (wall/roof junction, wall/floor junction, window reveal, lintel over openings, balcony connection). Thermal bridges are measured by their linear thermal transmittance (Ψ — 'psi' value, measured in W/mK).

Approved Document L requires that thermal bridges are assessed and minimised as part of the Part L compliance calculation. In practice, for a London extension, the critical thermal bridge details are:

*1. Window and door reveals*: Where a window frame is set in a cavity wall, the inner reveal (the inner face of the wall visible inside the room) is typically a return of plasterboard or block — not insulated. This creates a thermal bridge. Mitigation: apply 25–50mm PIR insulation to the inner reveal; ensure the frame overlaps the inner insulation at the edges.

*2. Roof-wall junction (parapet or eaves detail)*: Where a flat roof meets the external wall, the insulation of the roof must be continuous with the insulation of the wall — any gap creates a cold bridge. Well-designed details ensure the roof PIR board butts tightly to the wall insulation with no thermal break.

*3. Floor-wall junction (ground floor slab edge)*: At the perimeter of a ground floor slab, the edge of the insulation is exposed or poorly protected. A 50mm vertical PIR upstand (insulating the inner face of the edge beam or foundation wall to a depth of 500mm) significantly reduces edge heat loss.

*4. Lintels over openings (bifold door lintels)*: The concrete or steel lintel over a bi-fold or sliding door opening is a significant thermal bridge — steel in particular (λ = 50 W/mK) has very high thermal conductivity. Insulated cavity lintels (e.g., Keystone Hi-Therm or similar) with an insulated thermal break reduce this bridge significantly.

*How thermal bridges are assessed for Part L*: For simple extensions, a set of 'accredited thermal bridge details' (ATDs from the Concrete Block Association, BBA, etc.) can be used — these are pre-calculated Ψ-values for standard junction details that have been approved for use without bespoke calculation. For complex junction details (especially in Passivhaus or near-Passivhaus designs), bespoke calculation using THERM or PHYSIBEL software is required.

**Glazing limits — the 'limiting area' rule under Part L 2022**:

Approved Document L 2022 introduced a new limiting area rule for extensions: the total area of new windows, doors, and rooflights in an extension must not exceed 25% of the floor area of the extension, PLUS the area of any existing windows and doors removed as part of the extension works.

*Example*: A rear kitchen extension has a floor area of 20m². Under the 25% rule, the maximum new glazing area is 5m² — PLUS the area of the existing rear door (say 2m²) and any windows removed from the new opening into the extension (say 2.5m²). Total maximum glazing = 5 + 2 + 2.5 = 9.5m².

This rule was introduced specifically to limit the excessive glazing that was common in kitchen extensions (full-width bi-fold or sliding doors covering the entire rear wall). An extension with more than 25% glazing may still be permissible if a whole-building SAP energy calculation (produced by a BRE SAPS-accredited assessor) demonstrates that the overall dwelling energy performance still meets the notional building standard.

*Practical impact*: For a 5m × 4m extension (20m²), a full-width set of bi-fold doors at 5.5m wide × 2.1m high = 11.55m² of glazing. This exceeds the 25% limit by itself. In this scenario, the whole-building SAP route is required to justify the glazing area. Most London extension projects with large bi-fold or sliding door systems will need a SAP calculation for Part L compliance.

**The notional building comparison method (SAP for extensions)**:

• Where the simple backstop approach is insufficient (e.g., for large extensions or those with glazing exceeding 25% of floor area), a SAP (Standard Assessment Procedure) energy calculation is required. A SAP assessment:
• •Models the existing dwelling's energy performance
• •Models the extended dwelling with the proposed extension and specification
• •Confirms that the extended dwelling's energy performance (carbon intensity, primary energy rate) is no worse than a 'notional' extended dwelling built to the backstop standards

*Who produces SAP calculations?* SAP assessments for extensions are produced by BRE-accredited SAP assessors (also known as NDEA — National Domestic Energy Assessors). Cost: typically £250–£600 for a single-storey extension SAP calculation. The SAP assessor needs: the floor plan of the extension, the fabric specification (wall, floor, roof, glazing U-values), the existing house age/type/heating system, and the location (postcode — affects solar radiation data used in the calculation).

**Part L and airtightness for extensions**:

For new dwellings, Part L requires an airtightness test (air pressure test — measuring the number of air changes at 50 Pa pressure: q50 in m³/(h·m²) or n50 in air changes per hour). For extensions to existing dwellings, an airtightness test is NOT mandatory — but good detailing of the airtightness layer (usually the wet plaster on the inner face of masonry, or the internal plasterboard and its tape and mastic seal in timber frame) significantly affects thermal performance. London builders who tape and mast all internal plasterboard joints and provide continuous airtight details around all penetrations produce better-performing extensions — which may also allow thinner insulation to achieve the same overall energy performance in a SAP calculation.

**Part L requirements for loft conversions**:

A loft conversion that creates new habitable space must meet the Part L backstop values for the new elements:

• *New roof structure (insulation between and over rafters — warm roof)*:
• •Between rafters: 100mm PIR between 150mm rafters (leaving 50mm residual rafter depth below insulation for thermal mass effect) + 12.5mm plasterboard; U ≈ 0.25 W/m²K — does NOT meet the 0.15 backstop
• •Between rafters + over rafters: 80mm PIR between rafters + 50mm PIR over (on counter-battens) + tile batten + tile; U ≈ 0.15 W/m²K — at the backstop
• •Over rafters only (full warm roof): 150mm PIR directly over existing rafters + counter-batten + tile batten + tile; U ≈ 0.15 W/m²K

In practice: a loft conversion that insulates only between the rafters will typically NOT achieve 0.15 W/m²K unless using aerogel insulation or very high-performance PIR filling the full rafter depth. The combination of between-rafter + over-rafter insulation is the most practical route to the 0.15 backstop in a standard London loft conversion.

*Flat roof dormer*: The dormer roof must achieve 0.15 W/m²K — as per the flat roof warm roof guidance above, minimum 150mm PIR on the structural deck.

*Dormer cheek walls (side walls)*: The dormer cheek walls are new external walls and must achieve 0.26 W/m²K. Typically: 100mm timber studwork + full-fill mineral wool (λ = 0.035) between studs + VCL membrane + 12.5mm plasterboard; U ≈ 0.30 W/m²K — just above the backstop. Adding 50mm PIR boards on the warm side (between studs and plasterboard) brings this to ≈ 0.18 W/m²K.

*Loft conversion rooflights (Velux-type, in-roof)*: Must achieve the rooflight backstop of 1.80 W/m²K. Standard VELUX triple-glazed Centre-Pivot (GGL) windows achieve Uw = 1.1–1.3 W/m²K. Standard double-glazed VELUX: Uw ≈ 1.4–1.6 W/m²K. The backstop of 1.80 is achievable with modern double-glazed rooflights but triple-glazing is recommended.

**The 25% rule for existing dwellings — when renovating, replacing, or extending requires upgrades**:

• Approved Document L also applies when existing building elements are renovated and their area exceeds 25% of the element's total area. For example:
• •If more than 25% of a flat roof is being replaced (e.g., a full re-roofing job on an existing flat roof extension), the whole roof must be upgraded to meet the 0.15 W/m²K backstop
• •If more than 25% of the external walls are being re-rendered or re-clad, the whole wall element must meet 0.26 W/m²K

This rule catches homeowners who are doing a major renovation — not just a new extension — and is often overlooked.

**Common Part L failures in London extensions (findings at Building Control final inspection)**:

*1. Insufficient flat roof insulation*: The most common failure: 100mm PIR on a flat roof was compliant pre-2022 but fails the current 0.15 backstop. Always specify 150mm PIR minimum (or verify the SAP calculation accommodates a thinner specification through compensatory measures elsewhere).

*2. Insufficient floor insulation*: 75mm PIR below slab was compliant pre-2022 but fails the 0.25 backstop for ground floor. Always specify 100mm PIR minimum below the slab.

*3. Glazing area exceeding 25% of extension floor area with no SAP calculation*: Very common where a large set of bi-fold doors is specified for a modest-sized extension. Building Control will ask for a SAP calculation or require the glazing to be reduced. SAP calculations should be commissioned before drawings are submitted, not after Building Control raises the issue.

*4. Thermal bridge at window/door reveals unaddressed*: Unseen by the inspector during construction but can cause condensation and mould growth in service — and is technically a Part L compliance issue at the junction between wall and window.

*5. No product data sheets provided at final inspection*: Building Control inspectors require evidence that the specified products were actually installed. Retain insulation product data sheets (or cut from packaging), glazing BFRC certificates or manufacturer's Uw data sheets, and have these available at the final inspection.