SAP Calculations for Extensions: Understanding the 25% Rule (Without the Confusion)

Most homeowners planning an extension hear about the 25% glazing rule long before they understand what it actually means. It often gets described as a fixed cap on how much glass you are allowed to have, which leads to a lot of unnecessary worry. The real rule is simpler, more flexible and much easier to work with once you understand how SAP handles heat loss in extensions.

If you need SAP calculations for an extension, the rules often feel unclear. Our extension SAP service breaks the process down in a simpler way.

The points below break the rule down in a clear way and explain how compliance is measured. If you are considering a large set of bifolds, rooflights or a heavily glazed rear elevation, this will help you understand what is and is not a problem in Part L.

Why the 25% Rule Exists

An extension increases the thermal envelope of the home. The more glazing you add, the more heat loss the new space carries. To keep things in balance, Part L sets a simple starting allowance. The idea is not to restrict good design but to make sure the extension performs at least as well as the rest of the building.

The rule is only designed to control heat loss. It does not stop you from choosing a highly glazed design. It simply controls how that design is assessed.

The Extension 25% Rule Explained

The rule is this:

Your glazing allowance is 25 percent of the extension floor area plus the area of any existing openings removed as part of the work.

If your total glazing stays at or below that number, the extension can normally comply using a basic method. If you go over it, you can still comply, but you will need a different calculation approach.

Many people do not realise that the allowance includes the old doors or windows you remove when you open up the back wall. This can make the allowed glazing far higher than expected.

When the Extension Fits Within the 25% Rule

If the glazing comes in under the allowance, the compliance check is straightforward. This is called the Reference Method. It only requires the new walls, roof, floor and openings to meet the standard U values set in Part L. There is no modelling and no SAP calculation of the whole dwelling.

This is the simplest way to comply and is usually enough for modest rear extensions with standard window sizes.

When You Go Above the 25% Rule

This is where most of the confusion begins. Many people assume that exceeding the allowance means the design is not permitted. That is not how Part L works.

If the glazing is larger than the allowance, you move on to the area weighted U value method. This compares your proposed extension to a notional version of the same extension with reduced glazing and standard U values. If the overall heat loss of your design is no worse than the notional one, the extension complies.

This method offers a lot of flexibility. Improving the insulation of the walls or roof, upgrading the glazing or adjusting one weaker element is often enough to bring the heat loss back into line.

When Large Areas of Glazing Still Do Not Pass

For heavily glazed, architect led designs there is a final option. This is the whole dwelling SAP calculation. Instead of treating the extension in isolation, the assessor models the effect on the full property to show that the updated home still performs acceptably as a whole.

This is how very glazed extensions are usually approved. It allows the design to proceed while maintaining the overall performance of the home.

Why Some Designs Struggle

Most problems arise not from the amount of glazing but from assumptions. If the glazing performance is worse than expected, or the walls and roof are slightly weaker than the plans suggested, the margin can tighten quickly. Rooflights can also influence results more strongly than homeowners expect because of their angle and solar gain pattern.

Understanding these details early avoids last minute changes. A quick check of specifications during the design stage usually prevents the issue entirely.

Conclusion

The 25 percent rule is not a restriction on design. It is just the starting point for assessing heat loss in an extension. You can have large doors, multiple rooflights or a fully glazed rear elevation. You simply need the right calculation method to show compliance.

If you are unsure which route your project will need, or if you want to avoid the usual glazing and U value pitfalls, I can review your plans and confirm the best approach before anything is submitted. This keeps the build on track and avoids costly surprises at the end.

Most extensions pass easily once the correct method is applied.

If you want a SAP calculation for your extension, here’s where to start.