Handover & Completion · Mandatory · Completion

Air Permeability and SAP Calculation

The Standard Assessment Procedure calculation assessing the energy performance of a dwelling.

Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. Building Regs Part L remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.

Legal basis	Building Regs Part L
Status	Mandatory for new dwellings
Max air permeability	8 m³/hr/m² at 50 Pa
SAP version	SAP 10.2 current version
Test by	ATTMA-accredited tester

1. Air Permeability and SAP — Proving Energy Performance at Completion

The air permeability test and the Standard Assessment Procedure (SAP) calculation together provide the quantitative evidence that a new dwelling meets the energy performance requirements of Part L of the Building Regulations. The air permeability test measures how airtight the building envelope is, while the SAP calculation models the overall energy performance of the dwelling, taking into account the building fabric, heating systems, ventilation, lighting, and renewable energy technologies.

There is a critical relationship between the design SAP calculation and the final as-built SAP calculation. The design SAP is produced before construction starts and is based on the specification of the building as designed, including an assumed air permeability value. The final SAP is produced after construction is complete and incorporates the actual measured air permeability from the pressure test. If the actual air permeability is worse than the design assumption, the final SAP may not meet the Part L targets, which would mean the dwelling does not comply with the Building Regulations.

The air permeability test must be carried out by a tester who is a member of the Air Tightness Testing and Measurement Association (ATTMA) or an equivalent approved body. The test involves pressurising the building to 50 Pascals using a blower door fan and measuring the rate of air leakage through the building envelope. The result is expressed in cubic metres per hour per square metre of envelope area at 50 Pascals. The maximum permitted air permeability for new dwellings under Part L is 8 m³/hr/m² at 50 Pa, but design values are typically much lower to achieve Part L compliance.

Remediate before the final test

A poor air permeability test result may mean Part L compliance fails — remediation before final test is far cheaper than after. If a preliminary or indicative test reveals poor airtightness, identify and seal the leakage paths before the formal test is conducted. Common leakage points include service penetrations, window and door junctions, floor-to-wall junctions, and loft hatches. Remediation at this stage is straightforward and inexpensive compared to remediation after the building has been completed and handed over.

2. What the Records Contain

The air permeability test report and SAP calculation together form the evidence base for Part L compliance. Each element of the documentation should be checked for accuracy and completeness.

Field	What to check
Property details	The full postal address and plot number of the property tested and assessed. Confirm that the property details match the dwelling being completed and that the test and SAP relate to the correct plot.
SAP version	The version of the Standard Assessment Procedure used for the calculation. The current version is SAP 10.2. Confirm that the correct SAP version has been used, as required by the edition of Part L under which the building regulations application was submitted.
SAP assessor	The name and accreditation details of the SAP assessor who produced the calculation. The assessor must be registered with an approved SAP accreditation scheme. Confirm that the assessor's accreditation is current and covers the type of assessment carried out.
Design SAP	The design-stage SAP calculation, including the Target Emission Rate (TER), Target Fabric Energy Efficiency (TFEE), Target Primary Energy Rate (TPER), and the corresponding Dwelling Emission Rate (DER), Dwelling Fabric Energy Efficiency (DFEE), and Dwelling Primary Energy Rate (DPER). The design SAP must demonstrate that the dwelling as designed meets the Part L targets.
Air permeability test	The air permeability test report, including the measured air permeability result in m³/hr/m² at 50 Pa, the date of the test, the name and ATTMA membership number of the tester, and a description of the test conditions and methodology. The result must be within the maximum permitted value of 8 m³/hr/m² at 50 Pa and should be consistent with the design assumption used in the SAP calculation.
Final SAP	The as-built SAP calculation, updated with the actual air permeability test result and any other as-built changes. The final SAP must demonstrate that the dwelling as built meets the Part L targets (DER ≤ TER, DFEE ≤ TFEE, DPER ≤ TPER). This is the calculation from which the EPC is generated.
EPC lodgement	Confirmation that the EPC has been produced from the final as-built SAP calculation and lodged on the EPB Register. The EPC reference number and lodgement date should be recorded. The EPC must be lodged before the dwelling is sold, let, or occupied.
Building Log Book	For dwellings, a summary of the energy performance data should be included in the home user guide or equivalent documentation provided to the homeowner. This includes the SAP rating, EPC rating, heating system details, ventilation strategy, and guidance on how to operate the building efficiently to achieve the energy performance predicted by the SAP calculation.

3. Common Mistakes

Not conducting the air permeability test before the EPC is produced

The final SAP calculation, and therefore the EPC, must be based on the actual measured air permeability of the completed dwelling. If the EPC is produced before the air permeability test is carried out, it will be based on an assumed or design value, not the actual performance. This means the EPC may overstate or understate the energy performance of the dwelling. The correct sequence is: complete construction, carry out the air permeability test, update the SAP with the actual result, produce the EPC from the final SAP, and lodge the EPC on the register.

Using a non-ATTMA accredited tester

Air permeability tests for Building Regulations compliance must be carried out by a tester who is a member of the Air Tightness Testing and Measurement Association (ATTMA) or an equivalent approved body. A test carried out by a non-accredited tester will not be accepted by Building Control and the EPC assessor cannot use the result in the SAP calculation. Always verify the tester's ATTMA membership before commissioning the test.

Not providing the SAP calculation to the building owner

The SAP calculation is an important document that forms part of the building regulations compliance evidence. It should be provided to the building owner as part of the handover documentation, along with the EPC, the Building Regulations Completion Certificate, and the home user guide. The building owner may need the SAP calculation for future reference, including when making alterations to the property or when selling the property.

Not remedying poor airtightness before the formal test

If a preliminary air permeability test or visual inspection reveals poor airtightness, the leakage paths should be identified and sealed before the formal test is conducted. Common leakage paths include gaps around service penetrations (pipes, cables, ducts), poorly sealed window and door frames, gaps at floor-to-wall junctions, and poorly fitted loft hatches. Remediation at this stage is straightforward and relatively inexpensive. If the formal test result exceeds the design assumption, the final SAP may fail to meet Part L targets, requiring costly remediation or redesign of other building elements to compensate.

Not confirming the correct SAP version

The version of SAP that must be used depends on the edition of Part L under which the building regulations application was submitted. If the wrong SAP version is used, the calculation will not be accepted by Building Control and the EPC will need to be reissued. Before commissioning the SAP assessor, confirm which edition of Part L applies to the project and therefore which version of SAP is required. For projects submitted under Part L 2021, the required version is SAP 10.2.

4. Frequently Asked Questions

What is the current SAP version?▾

The current version of the Standard Assessment Procedure is SAP 10.2, which is used for projects submitted under Part L 2021 (Conservation of fuel and power). SAP 10.2 introduced significant changes from the previous version (SAP 2012), including updated carbon emission factors, a new primary energy metric, revised fuel prices, and changes to how renewable energy technologies are credited. The SAP version required for a specific project depends on the edition of Part L under which the building regulations application was submitted. It is essential to confirm the correct SAP version at the start of the project.

What does the air permeability test involve?▾

The air permeability test (also known as a pressure test or blower door test) measures how airtight the building envelope is. A large fan is temporarily installed in an external doorway using an adjustable frame and fabric panel. All intentional ventilation openings (trickle vents, extract fans, flues) are temporarily sealed. The fan is used to pressurise the building to 50 Pascals above atmospheric pressure, and the volume of air that must be pumped into the building to maintain that pressure is measured. This gives the air leakage rate, expressed in cubic metres per hour per square metre of building envelope area at 50 Pa. The test is then repeated in depressurisation mode. The result is the average of the pressurisation and depressurisation measurements.

What is the maximum permitted air permeability for new dwellings?▾

The maximum permitted air permeability for new dwellings under Part L of the Building Regulations is 8 m³/hr/m² at 50 Pa. However, this is the absolute maximum — in practice, design air permeability values are typically much lower (often in the range of 3 to 5 m³/hr/m² at 50 Pa) because a lower air permeability reduces heat loss and makes it easier to meet the Part L carbon emission and primary energy targets. If the design SAP calculation assumes a low air permeability, the building must achieve that value or better in the test, not just the regulatory maximum of 8.

Is air permeability testing required for all new buildings?▾

Air permeability testing is mandatory for all new dwellings under Part L 2021. For non-domestic buildings, testing is also mandatory for new buildings. There are limited exceptions for very small extensions and some types of building work where testing is not practicable, but for new-build construction the requirement is effectively universal. The test must be carried out on a sample basis for developments of multiple dwellings of the same type, or on every dwelling if the development consists of different dwelling types. The sampling regime is set out in Part L and the associated Approved Document.

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