Approved Document L 2026 – what’s changed?

Approved Document L was last updated in 2021 in preparation for the launch of the Future Homes Standard (FHS). Now that the FHS has arrived, the document has been overhauled and renamed. Approved Document L: Energy and Greenhouse Gas Emissions now covers everything from fabric performance to low-carbon heating and renewable energy requirements.

Here’s what’s changed in Approved Document L (ADL) 2026:

New requirements for low-carbon heating and renewables

Probably the most noticeable updates are the new minimum requirements for solar panels and low-carbon heating, particularly heat pumps. Although gas boilers are not banned outright, it will be difficult in practice to meet the emissions requirements in ADL 2026 without adopting low-carbon heating solutions.

On-site renewable electricity generation must also form part of every new building. ADL 2026 states that solar panels should cover an area equivalent to at least 40% of the building’s ground floor area. It would be easy to look at these major changes and assume the building fabric has become less important, but fabric performance should still be the first consideration.

New homes must meet three performance targets: the Target Emission Rate (TER), the Target Primary Energy Rate (TPER) and the Target Fabric Energy Efficiency rate (TFEE). Renewable technologies and low-carbon heating can help achieve the TER and TPER, but both are heavily influenced by the quality of the fabric. Lower heat demand improves overall efficiency and helps systems such as heat pumps operate effectively.

Greater scrutiny and new compliance requirements

Among the many tweaks, updates and new definitions in ADL 2026, there is a clear shift towards closer scrutiny of building performance. The Home Energy Model (HEM) will eventually replace SAP 10.3, allowing a more detailed assessment of building performance. Airtightness requirements have also become more demanding.

More information must now be provided to homeowners, including photographic evidence of build quality and signed BREL reports from both the design and as-built stages. Homes are also recommended to be “smart meter ready”. All of this points to greater accountability at every stage, from design through to completed construction and use.

Much finer margins for fabric performance

On the surface, fabric requirements have not changed dramatically. Maximum U-values for most thermal elements remain the same as in ADL 2021. However, the margin for error is far narrower.

Window U-values have tightened, and the guidance now requires a more comprehensive range of window components to be included in thermal calculations. The document also makes clear that meeting the TFEE target will often require performance levels significantly better than the limiting U-values.

For example, while it may still be theoretically possible to meet the new requirements with a 100mm cavity wall, doing so in practice will be difficult. Windows may need to be smaller and triple glazed, while material substitutions during construction could have a significant impact on the final U-value.

A 150mm cavity wall provides much greater resilience when projects do not go exactly to plan, along with more flexibility elsewhere in the specification. It also enables the use of full-fill insulation solutions such as glass mineral wool, which are easier to install correctly and can help maximise airtightness.

Interactions with other Building Regulations

ADL 2026 expands the number of what it describes as “key interactions” with other parts of the Building Regulations. This provides a useful starting point for understanding how compliance with Part L may be affected by wider regulatory obligations.

Fire safety is a good example. ADL 2026 highlights the need to consider fire performance due to the increased use of on-site electricity generation, particularly solar panels.

Fires associated with solar installations are a recognised risk and should be addressed during the design stage. The use of non-combustible insulation can help limit the spread of fire within the building fabric. Glass mineral wool, for example, is typically classified as A1 or A2-s1,d0 under the Euroclass reaction-to-fire system.1

Fabric is the foundation

ADL 2026 raises the bar across the board. Low-carbon heating, solar generation and tighter compliance requirements may grab the headlines, but fabric performance remains the foundation of a successful specification. With less room for error and greater scrutiny throughout the build process, getting the basics right early in the design stage will be critical for meeting the new standards efficiently and reliably.