Extensions & Part L Compliance
Frequently Asked Questions

Not always. If your extension meets the standard limits in Part L of the Building Regulations, no calculation is needed at all.

Under Approved Document L, an extension meets the standard limits if both of the following apply:

• The new walls, roof, floor, windows, doors and rooflights meet the minimum insulation standards (U-values) for new work. In England, that's currently 0.18 for walls and floors, 0.15 for roofs, 1.4 for windows and doors, and 2.2 for rooflights.
• The total glazing is within the 25% glazing rule (explained in the next question).

In that case, building control will simply check your specification, and you won't need an assessor.

A calculation is only needed when a design goes beyond those limits, which usually means more glazing than the rule allows. That's very common with modern extensions featuring bifold doors, glazed walls or roof lanterns.

If you're not sure which side of the line your extension falls on, send us your drawings and we'll happily take a quick look free of charge. If you don't need us, we'll tell you.

The total area of windows, doors and rooflights in an extension shouldn't be more than 25% of the extension's floor area, plus the area of any existing windows and doors that the extension covers up.

For example, a 20m² extension gets an allowance of 5m² (25% of 20m²). If building it means losing an existing 2m² back door and a 1.5m² kitchen window, those are added on, giving a total allowance of 8.5m².

For this rule, everything in an external opening counts, measured as the whole unit including its frame:

• Windows, including fixed glazed panels
• Bifold, sliding and French doors
• Solid external doors
• Rooflights, roof windows and roof lanterns

Going over the allowance isn't the end of the world. It just means one of the two calculation methods below will be needed to show the extension still complies.

You don't have to cut back your glazing. Approved Document L offers two recognised alternatives that let you show the design still complies.

Both work by comparing your extension against a "notional" version of the same size and shape, built to the standard limits with the full 25% glazing allowance. The question is whether your design performs at least as well overall:

• The area weighted U-value method compares the average heat loss of the extension itself.
• The whole dwelling method uses a SAP calculation to compare the energy performance of your whole home, including the extension.

In most cases, one of these methods will get the design through with little or no change. Where it doesn't, we'll help you find the most cost-effective adjustment, which is usually far less drastic than removing glazing.

For a typical single storey extension, we normally start with the area weighted U-value method, because it's simpler and cheaper. The whole dwelling method is used where that isn't enough, or for larger and more complex projects.

We'd usually go straight to the whole dwelling method for:

• Extensions of more than one storey
• Several pieces of work on the same home at once, such as a loft conversion and a rear extension, which can then be assessed together
• Designs where the area weighted method is clearly not going to pass

You don't need to work out which one you need. We'll recommend the most cost-effective method for your project, and we won't charge you for a more detailed assessment than it actually needs.

For Part L purposes, loft and garage conversions are handled in much the same way as extensions. Conservatories and porches are often exempt.

• Loft and garage conversions: technically, turning an unheated space into part of your heated home is a "change to energy status" rather than an extension. In practice, compliance is shown in the same way, and if the design goes beyond the standard limits, the same two calculation methods are used.
• Conservatories and porches: these are exempt from the Part L energy requirements if they're at ground level, no more than 30m², separated from the house by external-quality walls, windows or doors, and not heated by the home's heating system (Approved Document L, paragraph 0.14). If the separating wall or doors are removed, or the heating is extended into it, it's treated as an extension instead.

If your conversion creates a separate, self-contained home, such as a garage turned into an independent flat, different rules apply. See our SAP calculations for conversions FAQ.

It's a calculation showing that the overall heat loss through your extension's walls, roof, floor, windows and doors is no worse than it would be for a standard compliant extension of the same size and shape.

A U-value measures how easily heat escapes through part of a building (the lower, the better). Glass lets heat escape much more easily than an insulated wall, so extra glazing pushes up the extension's heat loss. The area weighted method adds up each element's U-value in proportion to its area, giving one average figure for the whole extension. That figure is then compared with the same calculation for a notional extension that meets the standard limits, with glazing at the full 25% allowance (Approved Document L, paragraph 10.9).

If your extension's average is the same or lower, it complies, even though it has more glazing than the standard rule allows.

It's a relatively quick calculation, based on your drawings and specification. In England, it only looks at the extension itself, so improvements to the rest of your home can't be counted. If those are needed to make up the difference, the whole dwelling method is the right route.

Usually, improving the performance of some parts of the extension is enough to balance out the extra glazing, without needing to reduce it.

Because the calculation is based on an average, any element that performs better than the minimum standard helps make up for the glazing. Depending on the design, that could mean better-performing windows, doors or rooflights, or extra insulation in the walls, roof or floor.

We'll show you which changes make the biggest difference for the least cost, so you can make an informed choice rather than guessing. If the area weighted method still can't get there, the whole dwelling method often can, and frequently without any changes at all.

The whole dwelling method is a SAP calculation of your entire home with the proposed extension, compared against your home with a notional compliant extension of the same size and shape.

Under Approved Document L, paragraph 10.10, your home as extended needs to perform no worse than the notional version on carbon emissions, primary energy use and fabric energy efficiency. SAP (the Standard Assessment Procedure) is the government's approved method for calculating the energy performance of homes.

It's typically used when:

• The area weighted method doesn't pass
• The extension is more than one storey
• Several pieces of work are happening at once, for example a loft conversion alongside an extension

Because it looks at the energy performance of the whole home rather than just heat loss through the extension, it quite often passes where the area weighted method fails, without any changes being needed.

Yes. This is one of the biggest advantages of the whole dwelling method. Improvements to your existing home can be counted towards offsetting the extra glass in your extension.

If the calculation doesn't pass at first, changes to the existing house, such as its heating, insulation or renewable energy, can be included in the assessment to close the gap. That means the answer doesn't have to be a compromise on the extension you actually want.

Every home is different, so there's no single standard fix. We look at your specific house and extension and recommend the most cost-effective option, which is sometimes something you were planning to do anyway.

Because the whole house is assessed, we need drawings or measurements of the existing home, along with whatever you know about how it's built and heated.

• Floor plans and elevations of the existing house. Existing drawings are ideal; otherwise, measurements taken by a competent person such as your architect or surveyor.
• The approximate age of the house, and what the walls, roof and floors are made of, including any insulation you know about.
• Window and door types, such as single, double or triple glazing.
• Heating, hot water and ventilation systems, plus any renewables such as solar panels or a heat pump.
• Photos are always helpful.

Don't worry if you don't know everything. Where the construction of the existing house is unknown, we use the standard default values from Appendix S of the SAP methodology, based on the age and type of the building. We work remotely from the information you provide.

No, not under the current Building Regulations. In England, Part O (overheating) only applies to new build homes, not to extensions.

This may change in future. The government has announced a review of Part O, which includes looking at large, heavily glazed extensions, although no date has been set.

It's also worth checking your planning decision notice, just in case it includes any conditions about overheating. If it does, we can help with that too.

The basic idea is the same across Great Britain, with a 25% glazing rule and ways to compensate for exceeding it, but the details differ. We work on extensions in England, Wales and Scotland.

• England: Approved Document L Volume 1, as described on this page.
• Wales: the Welsh Government's own Approved Document L offers two equivalent options, called the "U-value trade-off approach" and the "equivalent primary energy and carbon emissions target approach". One helpful difference is that in Wales, insulation improvements to the existing house can also be counted in the simpler U-value trade-off calculation.
• Scotland: Section 6 of the Scottish Building Standards Domestic Technical Handbook uses a similar 25% glazing rule and a "compensatory approach" that compares heat loss against a notional extension. A whole dwelling SAP route exists, but it's generally only practical where SAP information for the existing home is already available.

Minimum U-values also vary between the three countries. We'll make sure your extension is assessed against the right set of rules for where you're building.

Ideally at design stage, before your building control application and certainly before glazing is ordered. For the extension itself, we'll need drawings and a specification.

• Scaled floor plans, elevations and sections of the extension
• Sizes of all windows, doors and rooflights, including any existing ones being covered up or removed
• Construction details of the new walls, roof and floor, including insulation types and thicknesses
• U-values of the proposed windows, doors and rooflights, if known
• For the whole dwelling method, the information about your existing house listed above

Getting the calculation done early means any adjustments can be made on paper, before anything is ordered or built. When you accept a quote, you'll receive a full checklist of what's needed. Get an instant quote, or contact us for a no-pressure chat about your extension.