Part L 2021: What Builders Must Do to Stay Compliant

Part L changed in 2021 and the shift has been bigger than many builders expected. Insulation values tightened, airtightness became more important and there is now far greater emphasis on proving how the home was actually built. Most delays at completion are not caused by major design problems but by small details that no one used to worry about. The rules are much clearer when you understand what Building Control want to see at each stage.

Why these changes matter

The 2021 update was designed to close the gap between how a dwelling is expected to perform on paper and how it performs once it is built. To do that, Part L now relies heavily on evidence. A loft full of insulation and a good set of drawings are no longer enough. Builders need to show that the construction matches the design and that the dwelling meets its final performance targets.

This is where SAP comes in. Every new home requires a design stage calculation before work starts and an as built calculation before sign off. They are connected, but they do different jobs. If you want a clearer breakdown of the process, our SAP calculation service is the quickest way to understand what Building Control expect and when.

What needs to happen before work starts

Before a spade goes in the ground, Building Control must be satisfied that the design can meet the carbon, primary energy and fabric efficiency targets set out in Part L. This means the builder or designer must submit a Design SAP, a BREL or BRWL report and the drawings used to create them.

The important thing here is realism. A design that assumes a very low air permeability or a high-performance window that has not actually been priced can cause trouble later. If the Design SAP is built on optimistic assumptions, the as built version usually exposes the gap.

Construction stage responsibilities

Once work begins, Part L places more responsibility on the builder. Evidence now influences the SAP result almost as much as the specification itself. Insulation needs to be installed without gaps or compression, thermal bridges should follow the agreed details and any product substitutions need to be checked early. What used to be minor site decisions can now affect the final numbers.

Airtightness is another area where expectations have shifted. Many homes still test higher than the design value, often because air barrier lines are not planned in detail. A slightly weaker test result can push the dwelling close to the limit, especially if other elements have changed on site. Treating airtightness as a deliberate part of the build, rather than something tested right at the end, makes a significant difference.

Changes happen on most projects and there is nothing wrong with that, but the SAP model should be updated as soon as they occur. A different window supplier, a revised insulation thickness or a change to the heating system can all alter the heat loss picture. Catching these early is far easier than repairing a compliance shortfall at completion.

What Building Control need at the end

When the dwelling is finished, the assessor updates the model to reflect the actual construction. This includes the air test certificate, datasheets for glazing and insulation, photographs showing key thermal elements and the final heating and ventilation information. Without this evidence, the SAP will default to conservative values which often make the dwelling fail on paper even if the construction is sound.

The EPC must be lodged within seven days of completion, so the as built SAP cannot be left as an afterthought. Builders who record evidence throughout the build tend to have a far smoother sign-off because nothing needs to be recreated once the house is complete.

Avoiding the common pitfalls

The same issues appear again and again under the 2021 regulations. Insulation that does not match the specification, windows with a slightly weaker U value than expected, missing photographs of cavity insulation or thermal junctions, and air test results that drift from the design target. None of these are major problems on their own, but they can add up quickly.

The simplest way to avoid most of them is to keep the Design SAP and the as built reality aligned. When the specification changes, update the model. When insulation goes in, photograph it. When a heating system is swapped for a different model, tell the assessor. Treat SAP as part of the build rather than an end-of-job formality.

If you want a clear process that covers both stages and avoids the usual surprises, our SAP compliance support can step you through the requirements and help you keep Building Control informed throughout the project.