SAP Calculations Explained

SAP calculations are used to assess the energy performance of residential buildings and demonstrate compliance with the energy-efficiency requirements of the Building Regulations.

They are commonly required for new dwellings, properties created through conversion or change of use, and certain extension projects.

Although the final result is important, a SAP assessment can also be a useful design tool. When carried out early enough, it can help identify which parts of the proposed specification have the greatest influence on compliance and allow different options to be considered before construction begins.

What Does SAP Mean?

SAP stands for Standard Assessment Procedure.

It is the Government-approved methodology used to calculate the predicted energy performance of dwellings.

The calculation considers the dwelling as a complete system rather than simply looking at the amount of insulation provided.

Information used within a SAP assessment can include:

Building dimensions and exposed areas
Floor, wall and roof construction
U-values
Thermal bridging
Windows and doors
Glazing performance
Airtightness
Heating systems
Hot-water systems
Ventilation
Heating controls
Renewable technologies
Lighting
Orientation and solar gains

The resulting assessment is used to determine whether the dwelling meets the applicable Part L requirements.

When Are SAP Calculations Required?

SAP calculations are most commonly required for new dwellings.

They may also be required where an existing building is converted to create new dwellings or where there is a material change of use.

Extensions can also require a SAP-based assessment in certain circumstances, particularly where the proposed glazing exceeds the normal allowances within Approved Document L and an alternative compliance approach is being used.

The exact requirements depend on the type of project and the Building Regulations that apply.

Design-Stage SAP Calculations

For a new dwelling, the first assessment should ideally be carried out while the project is still at the design stage.

Architectural drawings and the proposed construction specification are entered into the SAP software to assess the dwelling before it is built.

This allows potential compliance issues to be identified while there is still an opportunity to review the specification.

If the initial assessment does not comply, the calculation can help identify the areas having the greatest effect on performance.

Alternative options provided by the project team can then be modelled to understand their effect before decisions are made.

Starting the SAP assessment early can therefore reduce the risk of discovering a significant compliance problem after the design has been finalised or construction has started.

What Happens if a SAP Calculation Does Not Pass?

A failed initial SAP assessment does not necessarily mean that every part of the specification needs upgrading.

Different elements of the dwelling influence the calculation in different ways.

For example, depending on the project, compliance may be affected by:

Insulation levels
Thermal bridging
Airtightness
Glazing performance
Heating efficiency
Hot-water performance
Ventilation
Heating controls
Renewable technologies

The useful part of the assessment is understanding which changes actually influence the result.

A SAP assessor can model alternative specifications provided by the project team and explain how those changes affect compliance.

This allows the designer, developer or client to make informed decisions rather than simply increasing specifications across the whole building.

What Information Is Needed for a SAP Calculation?

For an initial Design-Stage SAP assessment, we will normally need architectural drawings together with as much of the proposed construction specification as is available.

Useful information includes:

Plans, elevations and sections
Floor, wall and roof constructions
Proposed insulation and U-values
Window and door specifications
Heating and hot-water systems
Ventilation strategy
Proposed airtightness target
Thermal bridging information
Heating controls
Renewable technologies where proposed

Not everything needs to be finalised before the first assessment is carried out.

If some information is still undecided, send us what you have and we can advise what additional information will be required as the assessment develops.

What Is an As-Built SAP Calculation?

The Design-Stage SAP assessment is based on the proposed design.

The As-Built SAP assessment is completed at the end of construction using the final information for the dwelling as actually built.

This may include updated construction specifications, final product information, air leakage test results, heating and ventilation details and other completion information.

Part L photographic evidence is also collected during construction to provide evidence of key construction details before they are covered up.

The final information should therefore reflect the building that has actually been constructed rather than simply repeating the original design specification.

Is a SAP Calculation the Same as an EPC?

No.

The SAP assessment and the Energy Performance Certificate are closely related, but they are not the same document.

SAP is the calculation methodology used to assess the energy performance of the dwelling and demonstrate the relevant Part L compliance requirements.

For a new dwelling, the final On Construction Energy Performance Certificate is produced following completion of the As-Built SAP assessment and the required completion information.

This is why the SAP process normally begins during design and continues through to completion of the dwelling.

What Can Change Between Design Stage and As-Built?

It is common for specifications to develop during construction.

Products may change, heating systems may be altered or different insulation, glazing or ventilation products may be installed.

Where these changes affect the SAP assessment, the final calculation needs to reflect the actual construction.

Changes should therefore be communicated to the SAP assessor as the project progresses rather than waiting until completion.

This is particularly important where a proposed substitution could reduce the energy performance of the dwelling or affect one of the compliance targets.

Air Leakage Testing and SAP

Air leakage is one of the inputs used within the SAP assessment.

At design stage, an intended air permeability figure is included within the calculation.

For dwellings requiring an air leakage test, the measured test result is then used when preparing the final As-Built SAP assessment.

This means the airtightness performance achieved on site can directly influence the final SAP result.

Good airtightness should be considered throughout construction rather than treated as something that can simply be resolved when the test is carried out.

Part L Photographic Evidence

Photographic evidence is required during construction to record key details relating to the building fabric and services.

Many of the required details become hidden as the build progresses, so the photographs need to be taken at the appropriate construction stage before they are covered up.

The evidence forms part of the information reviewed when the final As-Built SAP assessment and compliance documentation are prepared.

Our Part L Photographic Evidence guide explains what should be photographed and how the records should be managed.

What About SAP 10.3 and the Future Homes Standard?

Projects need to be assessed using the methodology and Building Regulations requirements applicable to that particular project.

Projects proceeding under the existing Part L 2021 requirements may continue to be assessed using the applicable SAP 10.2 route, while SAP 10.3 supports the new Future Homes Standard requirements coming into force from March 2027, subject to transitional arrangements.

Our Future Homes Standard & SAP 10.3 guide explains the changes, transitional arrangements and what they mean for new residential projects.

When Should You Get a SAP Assessor Involved?

Ideally, before the construction specification is completely fixed.

The earlier the assessment is carried out, the greater the opportunity to identify potential issues and consider practical alternatives.

Leaving SAP until shortly before a Building Control submission or until construction has started can mean that fewer options are available if changes are required.

Early assessment is particularly useful on projects with unusual construction methods, large areas of glazing, challenging building forms or developing heating and renewable-energy strategies.

How Crowther Solutions Can Help

Crowther Solutions provides SAP calculations and residential Building Regulations compliance support for projects across the UK, with experience in construction and energy assessment dating back to 2005.

We provide both Design-Stage and As-Built SAP assessments and can support projects from the initial design through to completion.

Where an assessment identifies a compliance issue, we can explain the factors influencing the result and model alternative options provided by the project team.

Our role is to assess the proposed design and provide technical compliance guidance. Final design decisions, product selection and specification remain the responsibility of the appointed designers and project team.

Need a SAP Calculation?

Send us your drawings and the construction information currently available.

You do not need to wait until every part of the specification has been finalised. We can review the information, advise what else is required and provide a project-specific quotation.

Request a SAP Calculation Quote