Part L updates: What you need to know

Posted by Lignacite

Tags: Part L

Revised Part L standards were published in March 2026 as part of the latest update to Building Regulations for use in England. The changes form part of the Government’s wider push towards greater energy efficiency and lower-carbon emissions. This change will affect how new buildings are designed, specified and assessed.

In this article, we look at what has changed, what remains the same and what the update means for architects, developers and contractors.

What is Part L of the Building Regulations?

Approved Document L is a section of the Building Regulations focused on the conservation of fuel and power and minimising carbon emissions in buildings. It sets energy efficiency standards for new dwellings and other building types.

The new standards were published by HM Government’s Ministry of Housing in March 2026 and will take effect from 24 March 2027.

There are two volumes:

Volume 1 relates to dwellings
Volume 2 relates to buildings other than dwellings

For higher-risk buildings (generally those at least 18 metres high or with seven or more storeys) the updated requirements take effect from 24 September 2027.

What are the changes to Part L in 2026?

The updates to dwellings form part of the Government’s wider Future Homes Standard ambitions to reduce operational carbon emissions from new homes.

The 2021 Part L revisions were an interim step towards the Future Homes Standard and introduced a target to reduce carbon emissions from new homes by around 30% compared with previous standards.

The 2026 update continues that direction of travel. It places greater emphasis on:

low-carbon heating systems
on-site renewable electricity generation
airtightness
thermal performance
reducing thermal bridging

This is likely to mean the installation of technologies such as heat pumps and solar photovoltaic (PV) panels as part of new residential developments. Gas will no longer be a heating option.

Although the formal limit for air leakage stays unchanged at 8.0 m³/(h·m²) at 50 Pa, the 2026 guidance stresses that better airtightness will usually be needed to meet overall energy and carbon targets. Typical air permeability rates of 4.0 and below will become the norm.

What’s not changing?

Notional wall, floor, roof and window U-values remain broadly unchanged from the 2021 Part L update.

While some industry experts anticipated lower target U-values, the notional fabric standards have seen limited change.

How does Part L relate to the Future Homes Standard?

Approved Document L Volume 1 is part of the Government’s wider Future Homes Standard programme, which aims to improve the operational energy performance of new homes through measures including improved fabric efficiency, low-carbon heating systems and on-site renewable electricity generation.

The Future Homes Standard supports the transition towards lower-carbon housing by reducing reliance on fossil fuels and improving overall energy efficiency. In practice, this means new homes are expected to need less energy for heating and produce lower carbon emissions over their lifetime.

The revised Part L guidance is one of the main ways in which these changes are being introduced into Building Regulations, influencing how homes are designed, specified and constructed.

What the Part L update means for Architects and Builders

All new dwellings will need to meet the 2026 Part L standards from 24 March 2027, but there are staged start dates for higher‑risk buildings.

This means architects, developers and contractors will need to pay close attention to operational energy performance, construction detailing and overall thermal efficiency from the earliest design stages.

Will external walls be affected?

The wall U-value assumed in the notional dwelling remains unchanged at 0.18.

However, the revised notional dwelling assumptions place greater emphasis on airtightness, thermal bridging and continuity of insulation. As a result, designers may need to pay closer attention to junction detailing and construction quality in order to achieve compliance.

The guidance also highlights the importance of minimising heat loss through thermally bridged junctions, such as floor and wall connections, lintels, cills and so on. However, specifiers can call upon a range of masonry bridging Psi values provided by trade associations and product manufacturers. Their use will ensure heat loss through such junctions is minimised, thereby helping to meet the fabric efficiency standards required to comply with Part L requirements.

Is masonry construction affected by the Part L changes?

Much of the reduction in operational carbon emissions is expected to be driven by building services and renewable electricity generation.

However, fabric performance and construction detailing remain important to achieving compliance with Building Regulations. The guidance places continued emphasis on airtightness, thermal bridging and continuity of insulation.

Because notional fabric specifications are largely unchanged, the updated Part L guidance means that masonry construction will require little or no change to existing compliant specifications.

What is a ‘notional dwelling’?

In order to set the energy, carbon emission and fabric energy efficiency rates within Building Regulations, a theoretical building is used to set the baseline energy targets, such as the Target Emission Rate.

This is known as a ‘notional dwelling’ and the approach is widely used within UK energy efficiency regulations.

A notional dwelling is based on the same size and shape as the actual dwelling being assessed, but with fixed performance standards for:

fabric
ventilation
heating systems
renewable energy generation

National Calculation Methodologies, including the Standard Assessment Procedure (SAP) and the future Home Energy Model (HEM), are then used to calculate compliance metrics such as energy and carbon emissions against this benchmark.

The specification of the notional dwelling plays a major role in determining the overall ambition of the energy efficiency standards. If the specification is too demanding, construction costs may increase. If it is not ambitious enough, the regulations may be criticised for failing to drive meaningful improvements in energy performance.

It is important to stress that dwellings do not need to be built to the exact same specification as the notional dwelling. Instead, the notional dwelling is used to set the energy efficiency benchmark through SAP and HEM calculations. Designers and assessors can then choose how they achieve compliance in practice.