Quick answer: a new dwelling passes Part L if its air permeability is 8.0 m³/(h·m²) at 50 Pa or lower (1.57 at 4 Pa by pulse) — Approved Document L Volume 1, Table 4.1, unchanged in the 2026 edition. But the as-built SAP calculation has to be run on the measured figure, so a result worse than the design assumption can still fail the dwelling on its emission or fabric energy efficiency rate. And below 3.0 as built, Approved Document F classes the dwelling as highly airtight, which rules out natural ventilation.
Check a result
The numbers
What it means
What the result has to clear
Three separate tests, and a result can pass one and fail another.
- The Part L backstop. 8.0 m³/(h·m²) at 50 Pa for a new dwelling, from Table 4.1 of Approved Document L Volume 1. It is a limiting value, so it cannot be traded against better insulation or more PV — above it the dwelling is sealed and retested.
- The figure your calculation assumed. The as-built calculation must use the measured air permeability, and the dwelling's emission rate, primary energy rate and fabric energy efficiency rate must still be no worse than their targets. This is where most awkward results land: comfortably under 8, but a long way off the 3 or 4 the design was modelled at.
- The ventilation rules. Approved Document F calls a dwelling "highly airtight" if the design figure is below 5.0 or the as-built figure below 3.0. Natural ventilation is not listed as suitable for those dwellings — so a naturally ventilated house that tests very tight needs either expert advice or a continuous mechanical extract system fitting.
What the numbers usually mean
| Result, m³/(h·m²) at 50 Pa | Where it puts you |
|---|---|
| Below 3.0 — highly airtight as built | Comfortably inside the Part L backstop. Under Approved Document F this is a "highly airtight" dwelling, so natural ventilation is not an option: it needs continuous mechanical extract or MVHR. |
| 3.0 to 5.0 — tight | Inside the backstop with room to spare, and the sort of figure the Future Homes Standard notional dwelling is built around. Check the ventilation strategy matches. |
| 5.0 to 8.0 — compliant, but only just | Passes the Part L limiting value. Whether the dwelling passes overall depends on the figure your SAP calculation assumed — if the design figure was tighter, the as-built calculation has to be re-run with this one. |
| Above 8.0 — fails Part L | Above the limiting value in Table 4.1. The dwelling has to be sealed and retested; no amount of extra insulation or PV elsewhere buys it off, because the limiting value is a backstop, not a trade-off. |
Limiting values from Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, Table 4.1 and paragraph 4.20; the 2026 edition repeats them in Table 3.1 and paragraph 3.21. Ventilation thresholds from Approved Document F, Volume 1: Dwellings, 2021, Appendix A and Table 1.6.
The rules around the test
Pressure testing is a statutory duty in its own right — regulation 43 of the Building Regulations — not just an input to SAP. Since Part L 2021 the old sampling rules are gone: BRE's SAP Conventions put it plainly, an air pressure test should now be carried out on every dwelling, so there is no averaging across a plot type and no +2.0 penalty for an untested unit. The result has to reach the building control authority within seven days of the final test, and building control may accept a certificate from a tester registered by Elmhurst Energy Systems or ATTMA.
Frequently asked questions
What is a good air tightness test result?
Anything at or below 8.0 m³/(h·m²) at 50 Pa passes Part L's limiting value, but "good" depends on what your SAP calculation assumed. Most new dwellings are designed around 3 to 5, because that is where the fabric energy efficiency and emissions targets are comfortably met. Below 3 the dwelling counts as "highly airtight" under Approved Document F and must have continuous mechanical ventilation.
What is the maximum air permeability allowed under Part L?
8.0 m³/(h·m²) at 50 Pa for a new dwelling — Table 4.1 of Approved Document L, Volume 1 (2021 edition). If the test is done by the low-pressure pulse method the equivalent figure is 1.57 m³/(h·m²) at 4 Pa. The 2026 edition of Approved Document L keeps both numbers unchanged.
Does every new house need an air tightness test?
Yes. Under Part L 2021 the sampling rules were withdrawn and every new dwelling is tested — BRE's SAP Conventions put it plainly: an air pressure test should now be carried out on every dwelling. Testing itself is a statutory requirement under regulation 43 of the Building Regulations, and the result has to reach the building control authority within seven days of the final test.
My test result is worse than the design figure but still under 8. Is that a problem?
It can be. The as-built SAP calculation must be run with the measured air permeability, not the design assumption, and the dwelling's emission, primary energy and fabric energy efficiency rates still have to be no worse than their targets. A result of 6.5 against a design figure of 4.0 will usually push the calculation the wrong way, and something else in the specification has to make it up — or the dwelling is sealed further and retested.
Does a very airtight house need mechanical ventilation?
Yes, and this is where results surprise people. Approved Document F defines a "highly airtight" dwelling as one with a design air permeability below 5.0, or an as-built figure below 3.0, m³/(h·m²) at 50 Pa. Natural ventilation is not listed as suitable for those dwellings. If a naturally ventilated house tests below 3.0, Approved Document F says either expert advice is sought or a continuous mechanical extract system is installed — a genuinely awkward conversation at completion stage.
Who can carry out the test?
The Building Regulations name testers registered by Elmhurst Energy Systems or the Air Tightness Testing and Measurement Association (ATTMA) — regulation 43(4) lets building control accept a certificate from either. We are registered and test across the South and South West.
Related: air tightness testing · MVHR explained · DER and TER · fabric energy efficiency (TFEE and DFEE) · free U-value calculator.