Part L Energy Efficiency: U-Values, SAP Calculations & New vs Existing Buildings

Summary

Part L is the building regulation that most directly affects carbon emissions from the built environment, and it has been progressively tightened over successive revisions. The 2021 edition (which came into force for new dwellings in June 2022 and for existing dwellings in June 2023) introduced more stringent standards, including changes affecting heat pump installation, replacement windows, and extensions.

For tradespeople, Part L has practical implications on every job that touches the building fabric or heating systems. Fitting a new boiler, replacing windows, insulating a loft, adding an extension — all are regulated under Part L. Understanding the minimum standards, the difference between new build and existing dwelling requirements, and when a SAP calculation is required prevents both under-specification and unnecessary compliance cost.

SAP (Standard Assessment Procedure) is the UK government's methodology for rating the energy performance of new dwellings. It is calculated using software and produces the energy rating shown on an EPC (Energy Performance Certificate). All new dwellings must be SAP-assessed. Extensions and material renovations to existing dwellings may require an EPC update but do not typically require a new full SAP calculation unless the change is substantial.

Key Facts

• U-value definition: Thermal transmittance in W/m²K — the lower the number, the better the insulation
• Replacement windows: ≤ 1.4 W/m²K whole window U-value (England, 2022); rooflights ≤ 1.6 W/m²K
• Extension walls: ≤ 0.28 W/m²K (new build standard) for extension in contact with outside air
• Extension roofs (pitched): ≤ 0.16 W/m²K at rafter level
• Extension floors: ≤ 0.22 W/m²K (new build standard); ≤ 0.25 W/m²K for existing dwelling renovations
• Existing dwelling — improving external wall: Target ≤ 0.55 W/m²K when technically feasible; cavity fill target ≤ 0.55 W/m²K
• Existing dwelling — roof insulation: ≤ 0.16 W/m²K at ceiling level (loft insulation); applies when loft is accessible and insulation is being installed
• Boiler minimum efficiency: Gas condensing boiler SEDBUK 2009 rating ≥ 92% (ErP Band A minimum); non-condensing boilers no longer permissible in most cases
• New heating systems (new build after June 2025): Part L 2021 requires new dwellings to target Fabric Energy Efficiency Standard (FEES) — achieved through high-insulation fabric rather than relying on heating efficiency alone
• SAP assessment: Required for all new dwellings; conducted by a CIBSE-registered energy assessor using SAP 10.2 software
• EPC: Energy Performance Certificate required for all dwellings when sold, let, or newly constructed; rating A (best) to G (worst)
• FENSA/CERTASS: Window replacement self-certification schemes — installer must certify compliance with Part L
• Thermal bridging: Linear thermal bridges at junctions (e.g. wall/floor, wall/roof) must be accounted for; use PSI values from accredited construction details
• Solar gains: Part O (overheating) now limits solar gain in new dwellings — glazing area and orientation affect Part L calculation

Quick Reference Table

Detailed Guidance

Calculating Required Insulation Thickness

U-value = λ / d, where λ = thermal conductivity (W/mK) and d = thickness (m). Rearranging: d = λ / U-value.

Example: To achieve U-value of 0.16 W/m²K using mineral wool (λ = 0.038 W/mK): d = 0.038 / 0.16 = 0.2375m = approximately 240mm

In practice, use online U-value calculators (e.g. Knauf, Rockwool, NIBE) that account for the full wall or roof construction, including air films, plasterboard, timber, external finishes, and thermal bridging through structural elements.

Thermal bridging: A wall or roof filled with insulation at the right thickness may still fail the U-value target if thermal bridges (e.g. timber studs, rafters at 25% of width) are not accounted for. The weighted U-value includes both the insulation zone and the timber zone — typically increases the effective U-value by 10–20% compared to the insulation-only calculation.

Extensions — Fabric Compliance

For extensions to existing dwellings, Approved Document L Volume 2 (existing dwellings) requires the new fabric elements to meet the following notional specification:

• Walls: 0.28 W/m²K
• Roof (pitched): 0.16 W/m²K
• Floors: 0.22 W/m²K (if new floor construction)
• Windows: 1.4 W/m²K (whole window)

Area-weighted U-value approach: Where meeting the target for an individual element would be technically impractical (e.g. a conservatory with predominantly glazed walls), a compensating approach may be used — better performing elements compensate for weaker ones, provided the overall fabric performance meets the notional whole-building target.

Heating controls: Any extension that includes a new or upgraded heating system must also comply with the heating controls requirements in Approved Document L — typically requiring a room thermostat, TRVs on all radiators except the room with the main thermostat, and (for new systems) a programmer or smart controller. See also heating controls.

Replacement Windows — FENSA and CERTASS

When fitting replacement windows:

1. The windows must achieve a whole-window U-value of ≤ 1.4 W/m²K
2. If the installer is FENSA or CERTASS registered, they self-certify compliance and notify the Local Authority on the customer's behalf — no separate Building Control application is needed
3. If the installer is not scheme-registered, a Building Control notification (and inspection) is required
4. The installer must provide the customer with a certificate of compliance — needed for conveyancing

Double versus triple glazing: Standard double glazing with a low-E coating and argon-filled cavity achieves approximately 1.2–1.4 W/m²K. Triple glazing achieves 0.8–1.0 W/m²K — beneficial for Part O compliance and comfort but not required by Part L for replacement windows in existing dwellings.

New Boiler Installation

Installing a replacement gas boiler in an existing dwelling:

• Must be a condensing boiler (non-condensing boilers are not permitted except in genuine exceptional circumstances — very short flue runs, heat-only boilers in inaccessible locations)
• SEDBUK 2009 efficiency ≥ 92% (ErP Band A)
• Controls must meet minimum requirements: programmer, room thermostat, TRVs
• Gas Safe registered engineer must certify the installation — this is a legal requirement under Gas Safety Regulations, not just Part L

New build heating after June 2025: The Future Homes Standard transition means new dwellings must increasingly use low-carbon heating (heat pumps). Gas boilers in new dwellings may be restricted or prohibited — the regulatory position is transitioning. Check current AD L requirements.

SAP Calculations and EPCs

SAP (Standard Assessment Procedure) is required for:

• All new dwellings (including conversions to residential)
• Any change of use to dwelling
• Extensions that cross certain size thresholds (complex — consult energy assessor)

SAP is NOT required for routine work on existing dwellings — but an EPC must be obtained when the property is sold or let, and major improvements (loft insulation, new boiler) should be reflected in an updated EPC.

Finding a SAP assessor: Use an energy assessor registered with a competent person scheme such as Elmhurst Energy or Stroma. SAP calculations cost approximately £200–£400 for a typical new dwelling.

Design SAP vs As-Built SAP: New dwellings require both — a Design SAP predicts energy performance and is submitted with planning; an As-Built SAP is done after completion to confirm what was actually built.

Part L Exemptions and Limitations

Technically impractical: Where meeting a U-value target would cause condensation risk, damage the structure, or require works disproportionate to the scale of work being done, a lower standard may be acceptable. This is a genuine exemption but narrow — it is not a general cost exemption.

Heritage buildings: Listed buildings and buildings in conservation areas may not need to comply with Part L if doing so would unacceptably alter the character — for example, external wall insulation on a listed Georgian facade. See listed buildings.

Cost proportionality: For existing buildings, guidance allows that improvement work need not be required if the cost payback period exceeds 15 years. This applies to fabric improvements triggered by other work (e.g. reroofing does not automatically require roof insulation if payback would be very long).

Frequently Asked Questions

My customer wants to install a gas boiler in a new extension — is that allowed?

In an extension to an existing dwelling, yes — the existing heating system can be extended. However, if the property is being built or significantly renovated, the heating system and controls must meet Part L requirements. For new-build dwellings, the Future Homes Standard is moving away from gas boilers — check current AD L for the specific situation.

Does adding insulation to an existing loft require Building Regulations approval?

Adding insulation to an existing loft is controlled work under Part L when it is being installed as a consequence of other work, or as part of a major renovation. For a standalone loft insulation project on a private home, it is typically covered by the installer's competent person scheme registration if applicable, or may not require formal notification if it is simple loft insulation (quilt between and over joists) without any structural work. Check the specific situation — in practice, most domestic loft insulation companies self-certify or are covered by government scheme requirements.

What U-value do replacement windows need?

≤ 1.4 W/m²K whole-window U-value. This is achieved by standard double glazing with low-E coating and argon fill — the minimum acceptable product for a FENSA-certified replacement window. A reputable window supplier will confirm the rated U-value for any window they supply.

My extension has a lot of glazing — does it need to meet the standard U-value?

If the glazing area is very large, an area-weighted average U-value approach may need to be used, showing that the better-performing walls and roof compensate for the lower-performing glazed areas. Part L allows this trade-off up to a limit. The calculation can be done by an energy assessor or using the approved SAP/iSBEM tools.

Does fitting an ASHP (air source heat pump) need to comply with Part L?

Yes — heat pump installation is controlled work under Part L and also under Part G (for the hot water element) and Part P (for the electrical connection). The installer should be MCS registered for the heat pump installation and NICEIC/NAPIT registered for the electrical work. MCS registration also enables customers to receive Boiler Upgrade Scheme grants.

Regulations & Standards

• Building Regulations Approved Document L (Conservation of fuel and power) — current edition: Volume 1 (new dwellings, 2021) and Volume 2 (existing dwellings, 2021 / updated 2023)

• SAP 10.2 — current government methodology for energy rating

• Building Regulations Approved Document F (Ventilation) — ventilation requirements that accompany insulation improvements

• The Boiler Plus Regulations 2018 — additional controls requirements for boiler replacement

• Building Regulations Approved Document L — GOV.UK — Official AD L documents

• NIBE — U-Value Calculator — Free online U-value calculation tool

• CIBSE — SAP Guidance — SAP methodology and assessor registration

• heating controls — Boiler Plus legislation and heating controls requirements

• boiler selection — Boiler sizing and efficiency

• building regs overview — Overview of all Building Regulation parts

• part m access — Accessibility requirements for extensions

• listed buildings — Part L exemptions for listed buildings