SAP Calculations for Extensions: When Are They Required?

Do you need SAP calculations for a house extension?

The answer depends on the design of your extension and how compliance with Part L is being demonstrated.

In England, Part L sets energy efficiency requirements for building work, including extensions to existing homes.

For a typical extension, the building fabric and new windows and doors must meet the relevant performance requirements.

Where the design follows the standard guidance, a separate whole-dwelling SAP assessment may not be necessary.

However, additional calculations may be appropriate if:

• The extension includes substantial glazing.
• The proposed construction doesn't follow the standard elemental approach.
• Alternative insulation or energy efficiency measures are proposed to compensate for other design features.
• Building Control requests further evidence to demonstrate compliance.

A SAP calculation is one possible route to demonstrating compliance. It isn't automatically required simply because you're building an extension.

What are the Part L glazing rules for extensions?

One of the most common reasons for needing additional energy calculations is an extension with large windows, sliding doors, bifold doors or rooflights.

Under the standard approach in Approved Document L, the total area of windows, roof windows, rooflights and doors in an extension should generally not exceed:

25% of the extension's floor area, plus the area of qualifying existing windows and doors that are removed or are no longer exposed because of the extension.

This provides a straightforward way of limiting heat loss through the building's openings.

Example: A 30 m² extension

Imagine you're building a single-storey rear extension with a floor area of 30 m².

The basic 25% allowance would be:

30 m² × 25% = 7.5 m²

If the extension also covers an existing external window and door with a combined area of 3 m², which are no longer exposed to the outside, the total allowance could be:

7.5 m² + 3 m² = 10.5 m²

This is a simplified illustration of the guidance.

The actual calculation depends on the proposed openings, the treatment of existing windows and doors, and the regulatory requirements applying to your project.

What counts towards the glazing allowance?

The calculation considers the relevant openings in the extension, including:

• Windows
• Roof windows and rooflights
• Bifold and sliding glazed doors
• Other external doors

The commonly used expression "25% glazing rule" is therefore slightly misleading because the guidance concerns the relevant window and door areas, not just the area of glass.

What happens if your extension exceeds the 25% glazing allowance?

Exceeding the standard opening-area allowance doesn't necessarily mean you must reduce the glazing or abandon the design.

Part L provides alternative approaches that can offer greater flexibility.

The objective is to demonstrate that the proposed extension achieves an appropriate level of energy efficiency despite the additional openings.

Depending on the project and the applicable regulations, this may be demonstrated through improved building fabric performance, an area-weighted U-value calculation or a whole-dwelling SAP assessment.

For example, an extension with large bifold doors and several rooflights might use better insulation, improved glazing or other suitable measures to compensate for the additional heat loss.

The most appropriate approach depends on the design.

It is worth considering these options before making expensive changes to architectural drawings or glazing specifications.

Three common ways to demonstrate Part L compliance

There are three main approaches described in England's Part L guidance for extensions.

1. Standard elemental approach

This is normally the simplest route.

The new walls, roof, floor, windows and doors meet the relevant performance standards, and the proposed openings remain within the permitted area allowance.

Where the extension satisfies the applicable requirements, a separate whole-dwelling SAP calculation may not be needed.

This approach is often suitable for straightforward extensions with conventional construction and moderate glazing.

2. Area-weighted U-value calculation

An area-weighted U-value assessment can provide greater flexibility.

Rather than considering each part of the extension in isolation, this approach compares the overall heat-loss performance of the relevant building elements against a compliant reference extension of the same size and shape.

For example, if a design includes additional glazing, improved insulation or better-performing windows may help compensate for the additional heat loss.

This approach assesses the combined thermal performance of the relevant building elements.

It is not necessarily the same as undertaking a full whole-dwelling SAP calculation.

3. Whole-dwelling SAP calculation

Where greater design flexibility is required, a whole-dwelling SAP assessment may be appropriate.

This considers the energy performance of the existing dwelling together with the proposed extension and compares it with a suitable compliant reference arrangement, following the applicable methodology.

Depending on the design, improvements to the existing property may be considered as part of the compliance strategy.

These might include upgrading insulation, improving glazing or making other energy efficiency improvements.

A whole-dwelling assessment can therefore provide a way of evaluating proposals that do not fit neatly within the standard elemental guidance.

The correct approach should be established before commissioning calculations, as a simpler assessment may be sufficient for some extensions.

Which types of extension commonly need additional calculations?

Extensions with bifold or sliding doors

Large glazed doors are popular for rear and kitchen extensions, particularly where the intention is to connect the living space with a garden.

However, extensive glazed openings can mean the design exceeds the standard Part L opening-area allowance.

Additional calculations may be needed to demonstrate that the proposal achieves an acceptable level of energy efficiency.

Extensions with large rooflights

Rooflights can provide useful daylight, particularly in deep single-storey extensions.

However, the relevant rooflight areas contribute to the overall opening-area assessment.

An extension containing both extensive rear glazing and several rooflights can exceed the standard allowance relatively easily.

Open-plan kitchen and living extensions

Open-plan extensions frequently combine large glazed doors, rooflights and substantial changes to the rear of an existing house.

These features can create a need for additional energy calculations, particularly where the proposed design relies on a substantial amount of glazing.

Two-storey extensions

Two-storey extensions are not automatically subject to full SAP calculations.

Their compliance requirements depend on the proposed construction, openings and chosen method of demonstrating compliance.

Additional assessment may be needed where the design includes unusual construction or departs from the standard guidance.

Extensions to older properties

Extensions to older homes can involve retained solid walls, limited existing insulation and other construction characteristics that need careful consideration.

Where a whole-dwelling assessment is proposed, the energy performance of the existing property may influence the calculation and the options available.

What information is needed for extension SAP calculations?

The information required depends on the compliance method being used.

For an initial review, the most useful documents are usually the architectural drawings showing the proposed extension and its relationship with the existing house.

These may include:

• Existing and proposed floor plans
• Proposed elevations and sections
• The floor area of the extension
• Window, door and rooflight sizes
• Details of existing openings that will be removed or enclosed
• Wall, roof and floor construction specifications
• Insulation details and U-values, where available
• Heating and ventilation information, where relevant
• Relevant information about the existing dwelling

Where a whole-dwelling SAP assessment is needed, additional information about the existing house may be required.

You don't need every technical detail finalised before requesting a quotation.

You can submit the drawings you currently have and explain which specifications are still being developed.

For a more detailed checklist, see our guide What Information Do You Need for a SAP Calculation?

How much do SAP calculations for extensions cost?

At SAP Calculations Online, our current indicative base prices for extension-related assessments are:

These prices provide an initial guide to the likely assessment cost.

The final quotation will depend on the type of calculation required, the complexity of the extension and the information available.

Additional U-value calculations are currently priced at £50 each where required. Priority turnaround may also be available for an additional £50.

Not every extension requires the same type of assessment, so we'll review the project before confirming the scope and price.

You can also visit our SAP Calculation Cost & Prices page for the full interactive pricing guide.

Indicative pricing last reviewed: October 2026.

When should you arrange SAP calculations for an extension?

Ideally, any necessary calculations should be considered during the design stage, particularly before large glazed openings or construction specifications are finalised.

This is especially important if:

• Your extension includes extensive glazing.
• Building Control has raised concerns about Part L compliance.
• You're considering compensatory insulation improvements.
• The design team wants to establish the appropriate compliance route before construction.

Early assessment can help avoid situations where glazing or other construction elements need to be changed after products have been ordered or building work has started.

If construction is already underway, it may still be possible to carry out the necessary calculations, although the available options could be more limited.

Do SAP calculations for extensions produce an EPC?

Not normally.

A typical extension to an existing dwelling does not, simply by being constructed, require a new Energy Performance Certificate.

The purpose of an extension-related SAP assessment is generally to help demonstrate compliance with the relevant Building Regulations.

This differs from the process for a new-build dwelling, where SAP assessments and new-build EPC certification are closely connected.

There may be other circumstances in which an EPC is required, but it should not automatically be assumed to form part of an extension SAP quotation.

What about the 2026 changes to Part L?

Building Regulations in England are changing, with the 2026 edition of Approved Document L published ahead of implementation.

The principal changes are due to take effect from 24 March 2027 for most building work, subject to transitional arrangements and different commencement provisions for certain higher-risk building work.

The updated guidance continues to provide different ways of demonstrating compliance for extensions, including elemental, area-weighted and SAP-based approaches.

The exact requirements depend on which regulatory standards apply to the project.

If your extension is approaching a regulatory changeover, it is worth confirming the applicable requirements with your designer, assessor or Building Control before finalising the specification.

Frequently asked questions

Do all house extensions need SAP calculations?

No. Many straightforward extensions can demonstrate Part L compliance by meeting the relevant building fabric, glazing and other standards. Additional calculations may be required where an alternative compliance approach is needed.

Do I need SAP calculations for an extension with bifold doors?

Possibly. Large bifold doors can contribute to an extension exceeding the standard opening-area allowance. The requirement depends on the total area of openings and the proposed compliance approach.

Can I have more than 25% glazing in my extension?

Yes, potentially. The standard guidance allows a qualifying additional area for existing openings that are removed or no longer exposed. Alternative compliance methods can also provide design flexibility where the standard allowance is exceeded.

Does the 25% glazing allowance include rooflights?

Yes. The relevant opening-area guidance covers windows, roof windows, rooflights and doors, not just vertical glazing.

What if Building Control has requested a SAP calculation?

Provide the relevant Building Control comments, together with your architectural drawings and any available specifications. These will help establish what calculations are required and whether a simpler compliance method may be suitable.

How much does an extension SAP calculation cost?

Our indicative base prices start at £150 for a small, straightforward extension and £225 for a larger extension or small conversion. The final price is confirmed after reviewing the project and the assessment requirements.

Can SAP calculations be completed after an extension is built?

Potentially, although retrospective assessments may require additional information about the construction and specifications. It is generally preferable to establish compliance during the design stage.