Part L 2021 for New Builds: What Homeowners Need to Know

What is Part L?

Part L covers conservation of fuel and power. For new dwellings, Part L1A applies. Compliance is demonstrated through SAP (Standard Assessment Procedure) calculations showing the dwelling meets Primary Energy Rate and CO2 Emission Rate targets.

The 2021 update requires new homes to produce 31% less CO2 than 2013 standards. This typically requires photovoltaic (PV) solar panels and high performance building fabric.

Key Part L 2021 Requirements

Fabric standards: maximum U values for walls (0.18 W/m²K), floors (0.13 W/m²K), roofs (0.11 W/m²K), and windows (1.2 W/m²K). Air permeability targets are typically tightened to 5 to 8 m³/(h·m²).

Heating systems: Gas boilers are still permitted but increasingly difficult to comply with. Heat pumps and hybrid systems are favoured. Mechanical ventilation with heat recovery (MVHR) is now standard.

What Are SAP Calculations?

SAP calculations are required at two stages: design stage (before building control approval) and as built (after construction). Design stage SAPs should be done early to avoid costly redesigns.

As built SAPs incorporate the air permeability test result and any specification changes. Building control won't sign off without a compliant as built SAP.

What Is the Future Homes Standard?

The 2021 update is an interim step. The full Future Homes Standard (expected 2025) will require 75 to 80% lower CO2 emissions than 2013 standards, effectively prohibiting gas boilers.

The Home Energy Model (HEM) will replace SAP as the calculation methodology. HEM uses dynamic modelling for more accurate energy predictions.

How Do You Achieve Part L Compliance?

Key strategies: high performance fabric (insulation, airtightness), efficient heating (heat pumps preferred), renewable energy (PV panels), and smart controls (zoning, weather compensation).

We design all new builds to exceed current Part L requirements, future planning against the 2025 Future Homes Standard.

How Does Part L Relate to the Other Approved Documents?

Part L is only one of the Approved Documents that a new home must comply with. Part F covers ventilation, Part O sets requirements to limit overheating and Part S requires electric vehicle charge points in new homes. Part B covers fire safety and Part M covers access. They interact: for example, a highly insulated, airtight house needs well-designed ventilation under Part F and careful glazing choices under Part O.

Your designer and builder should look at all of these together at the design stage, because fixing a conflict once the building is under construction is far more expensive.

Does Part L Apply to Extensions?

Yes. Part L also applies to extensions and conversions, though the requirements are set out differently from new homes. New elements such as walls, roofs, floors and windows must meet minimum thermal performance standards, and new heating and lighting installed as part of the work must be efficient. Your Building Control body will check this as part of the approval.

Next Steps

We build new homes to current Building Regulations, and can talk through the process from design to completion. Call 020 4569 0526 or use our contact page.

Sources and Further Reading

Approved Document L, conservation of fuel and power: https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l

All Approved Documents: https://www.gov.uk/government/collections/approved-documents