How to comply with Part L Building Regulations

What Part L is trying to achieve

Part L aims to reduce the energy used and carbon emitted by buildings. For dwellings it sets targets for the whole home as well as minimum standards for individual elements such as walls, windows and heating systems. Recent updates have raised performance expectations, and low-carbon heating and on-site generation are becoming increasingly common in compliant designs.

Every project is different, so the right combination of measures depends on the building, its form and the client's priorities.

Fabric performance

Walls, floors and roofs with good U-values reduce heat loss and give the rest of the design more room to work with. Fabric is also difficult to improve after construction, so it is usually worth getting right from the start. Compact building forms with fewer exposed surfaces also tend to perform better.

Airtightness

Uncontrolled air leakage can account for a meaningful share of heat loss. The SAP calculation uses a design air permeability, and new dwellings are typically tested on completion to confirm it. Choosing a realistic target, and detailing the airtight layer properly on the drawings, helps avoid a gap between the design and as-built results.

Ventilation

As homes become more airtight, a suitable ventilation strategy becomes more important for indoor air quality. Options range from intermittent extract fans to continuous mechanical extract or mechanical ventilation with heat recovery. Each affects the SAP result differently, so the choice should be made with both compliance and the occupants in mind.

Heating and hot water

The heating system often has the largest single effect on the result. Heat pumps typically perform well under current SAP, while direct electric or fossil-fuel systems may need stronger fabric or renewables to compensate. Controls, emitter type, hot water cylinder size and insulation, and pipework all play a part.

Windows and glazed areas

Windows usually lose more heat than the surrounding walls, so the area, orientation and U-value of glazing matter. Large glazed areas can work well but may need better-performing units or compensating improvements elsewhere. Solar gain from glazing can help in winter, while for new residential buildings within the scope of Part O, overheating risk is considered separately.

Thermal bridging

Heat escapes at junctions such as lintels, sills, wall-to-floor and wall-to-roof connections. Where junction details are not specified, SAP applies cautious default values that can noticeably worsen the result. Using recognised construction details, or calculated values for the actual junctions, often gives a better and more accurate outcome.

Renewable and low-carbon technologies

Solar PV is a common way to improve the primary energy and carbon results, particularly where other options are limited. Heat pumps, solar thermal and waste water heat recovery may also help. The benefit depends on system size and how it interacts with the rest of the design.

Why early SAP assessment helps

A design-stage SAP calculation shows whether the proposals are likely to comply and which changes would be most effective. Making those decisions before drawings are finalised and orders are placed avoids costly redesign later.

If you want to know more about who carries out our assessments, see About us.