Part L Changes Under the Future Homes Standard

Part L (Conservation of Fuel and Power) of the Building Regulations sets the energy performance standards for new buildings in England. Under the Future Homes Standard, Part L is receiving its most significant update in decades, requiring at least 75% lower carbon emissions than homes built to the 2013 baseline, effectively ending gas heating in new builds and setting fabric standards that will make high-performance glazing and mechanical ventilation the norm.

The Journey: Part L 2013 → 2021 → FHS

The Future Homes Standard represents the third step in a progressive tightening of Part L standards. The table below compares notional dwelling values (the targets designs must meet), not backstop limits.

The 2021 uplift (which came into force in June 2022) was explicitly described as an interim step towards the Future Homes Standard. It tightened fabric standards and introduced primary energy as a compliance metric, but still permitted gas boilers. The FHS goes much further.

Fabric Specifications

The FHS notional dwelling carries its fabric U-values over from the 2021 uplift largely unchanged; the airtightness assumption tightens. The FHS's carbon and energy savings come principally from the services specification: a heat pump, decentralised mechanical extract ventilation, wastewater heat recovery, and the separate renewable electricity requirement. The fabric values below are not prescriptive requirements (the whole-building approach allows trade-offs), but they indicate the level of fabric performance the targets are built on.

External Walls: 0.18 W/m²K

The notional wall U-value is 0.18 W/m²K, unchanged from Part L 2021. The backstop (limiting) value remains 0.26 W/m²K. Wall constructions that comply today (full-fill cavities, insulated timber frame) continue to comply under the FHS, though the fabric energy efficiency target rewards going better than notional where the design allows.

Ground Floors: 0.13 W/m²K

The notional floor U-value is 0.13 W/m²K, unchanged from Part L 2021. The backstop remains 0.18 W/m²K. This typically requires 100–150 mm of PIR insulation below the slab or within a beam-and-block floor system, depending on the floor construction and perimeter-to-area ratio.

Roofs: 0.11 W/m²K

The notional roof U-value is 0.11 W/m²K, unchanged from Part L 2021. The backstop remains 0.16 W/m²K. For pitched roofs, this means mineral wool at joist level of approximately 350–400 mm, or equivalent performance from rigid board insulation in warm-roof constructions.

Windows and Doors

The notional window U-value is 1.20 W/m²K, unchanged from Part L 2021, and the notional door U-value is 1.0 W/m²K. High-performance double glazing can achieve 1.2 W/m²K, but many builders specify triple glazing (typically 0.8–1.0 W/m²K whole-unit) for the compliance margin it buys elsewhere in the calculation, offering:

• More consistent performance across different frame types and sizes
• Greater margin for compliance without relying on narrow product selections
• Improved acoustic performance and reduced condensation risk
• Better solar control, helping manage Part O overheating requirements

Glazing Area Limits

The notional building matches the actual dwelling's glazing up to a cap of 25% of total floor area. Where the actual design exceeds that, the notional building's windows are resized to the cap and the difference is replaced with opaque fabric, so glazing beyond 25% has to earn its keep against a better-performing reference. Notional glazing g-values follow the actual dwelling, to avoid conflict with Part O overheating design.

These glazing limits interact with Part O overheating requirements and larger glazing areas, particularly south and west-facing, increase the risk of overheating in well-insulated homes.

Airtightness

The FHS notional dwelling assumes an airtightness of 4 m³/(h·m²) at 50 Pa, tightened from the Part L 2021 notional value of 5 m³/(h·m²). The backstop (limiting) value remains 8 m³/(h·m²). With typical new builds currently achieving 4–6 m³/(h·m²), the notional value sits at the better end of current practice, and any dwelling testing worse than 4 must make the shortfall up elsewhere in the calculation.

Achieving 4 m³/(h·m²) consistently requires:

• Continuous air barriers designed and detailed from the outset, not applied retrospectively
• Factory-finished cassette or panelised construction methods that deliver consistent airtightness
• Careful service penetration design,minimising and sealing all penetrations through the air barrier
• Pre-completion pressure testing to verify performance before handover

At this level of airtightness, mechanical ventilation becomes essential to maintain indoor air quality. The FHS notional dwelling specifies dMEV (decentralised mechanical extract ventilation), though many builders choose MVHR for its heat recovery benefits. The interaction between airtightness and ventilation strategy is critical to FHS compliance. See our Ventilation & Part F page for full details.

Heating Requirements

The FHS represents a fundamental shift in how new homes are heated. The carbon emission targets are set at a level that no fossil fuel heating system can achieve, making low-carbon heating a de facto requirement.

Heat Pumps as Standard

Air source heat pumps (ASHPs) will be the default heating technology for the majority of new homes. Ground source heat pumps (GSHPs) and shared ground loop systems are also acceptable and offer higher efficiencies, particularly in larger developments.

Heat pump design under HEM is more demanding than under SAP because the model calculates variable COP (Coefficient of Performance) at each half-hourly timestep based on actual source and sink temperatures, rather than using a simplified seasonal average. This means:

• Flow temperatures matter more: lower flow temperatures (35–40°C) deliver higher COPs, favouring underfloor heating or oversized radiators
• Product selection is critical: HEM requires specific make/model data from the Product Characteristics Database (PCDB)
• Sizing must be accurate: undersized heat pumps running on back-up electric resistance heating will be penalised heavily in the calculation

Heat Networks

Heat networks with low-carbon sources are accepted as an alternative to individual heat pumps. This is particularly relevant for higher-density developments and apartment buildings where individual ASHPs are impractical. The heat network must demonstrate that the heat source meets the required carbon intensity.

Systems That Will Not Comply

The following heating systems cannot meet, or will struggle to meet, the FHS carbon targets:

• Gas boilers (including condensing models)
• Oil boilers
• Hybrid boilers (gas boiler + heat pump combination)
• Hydrogen-ready boilers (a system that can burn more than one fuel is assessed on its highest-emission fuel, so these are assessed as natural gas)
• Direct electric heating (resistive heaters, storage heaters) is not barred at fuel level, but with a COP of 1.0 against a notional heat pump it will struggle to meet the TER and TPER in most dwellings

Hot Water Changes

HEM models hot water demand very differently from SAP. Instead of a simple monthly estimate based on floor area, HEM uses individual tapping events (each shower, bath, and tap use) throughout the day, with stratified cylinder modelling that tracks temperature layers within the hot water storage tank.

This means the FHS assessment requires far more detailed data about the hot water system:

• Individual tap flow rates and shower types (mixer, instantaneous electric)
• Pipework lengths and insulation between the cylinder and each outlet
• Cylinder specifications including volume, insulation thickness, and heat loss characteristics
• Heat pump connection details for hot water production

The practical consequence is that homes with hot water cylinders (essential for heat pump systems) are modelled more accurately, and well-designed systems with short, insulated pipe runs are properly rewarded.

Solar PV Requirements

In June 2025, the government confirmed that solar panels will become a functional requirement of the Building Regulations under the FHS. The key requirements:

• PV coverage equivalent to 40% of the dwelling's ground floor area where feasible (AD L1 para 5.73)
• A “reasonable amount” where shade, orientation, or roof geometry constraints apply
• The requirement does not apply where no reasonable output of renewable electricity is possible on-site or within the curtilage

HEM's half-hourly modelling is particularly well-suited to assessing solar PV. Unlike SAP's annual yield estimate, HEM models generation and self-consumption at each timestep, properly rewarding:

• Optimal panel orientation and tilt
• Battery storage that shifts generation to evening demand
• Heat pump scheduling to use daytime solar generation
• Reduced grid import, reflected in both carbon and cost calculations

Thermal Bridging

Under HEM, thermal bridges are calculated at every half-hourly timestep rather than as a static annual addition. This means thermal bridging has a larger impact on the overall calculation, particularly in colder weather when heat loss through junctions is greatest.

Thermal bridge-free design, using proprietary junction details or continuous external insulation systems, becomes increasingly valuable under FHS. The alternative is to model each junction individually using thermal modelling software (to BS EN ISO 10211), which adds significant assessment cost.

Part O Interaction: Overheating

The FHS creates a tension between minimising heat loss (Part L) and preventing overheating (Part O). Highly insulated, airtight homes with large south-facing windows can overheat in summer, and HEM's half-hourly simulation captures this dynamic in detail.

Architects must balance:

• Glazing strategy: south-facing glazing provides beneficial winter solar gains but increases summer overheating risk
• Shading: external shading (brise-soleil, deep reveals, overhangs) is highly effective; internal blinds are less so
• Ventilation: purge ventilation (opening windows) is the primary summer cooling strategy, but may conflict with noise requirements in urban areas
• Thermal mass: HEM's dynamic modelling properly credits thermal mass for absorbing daytime heat and releasing it at night

Compliance Methodology

Part L compliance under the FHS is demonstrated by comparing the proposed dwelling against a notional building of the same size and shape. The same notional building specification (see the notional building specification table) applies under both compliance routes; it is published in a format for each methodology.

Two compliance routes are available during the transition:

• HEM: FHS assessment via the ECaaS platform, the primary route, using half-hourly simulation for the most accurate results
• SAP 10.3, an updated version of SAP retained as an interim alternative during the dual methodology period

For full details on both routes and how to choose between them, see our Compliance Pathways page.

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