Part L Compliance · Elmhurst Accredited
A failed air test stops a handover. We test to Part L of the Building Regulations for new builds and conversions, and as quality assurance on retrofit works — with a pre-test checklist that catches the usual culprits before test day rather than on it.
An air permeability test measures how much air leaks through the building envelope under a controlled pressure difference. A calibrated fan is sealed into a doorway, the building is pressurised to 50 pascals, and the airflow needed to hold that pressure tells you how leaky the envelope is.
The result is expressed as m³/(h·m²) at 50 Pa — cubic metres of air per hour, per square metre of envelope area. Lower is tighter. Part L sets the maximum permitted figure, and the as-built result must also be consistent with the figure assumed in the SAP calculation used for the compliance submission.
The trap that catches people: a plot can pass Part L outright and still fail its own SAP assumption. If the design assumed 3.0 and the plot tests at 4.5, the SAP has to be re-run — and the dwelling may then fail on carbon or fabric energy efficiency even though the air test itself was within the regulatory limit.
We flag that at the point of testing, not after your building control submission comes back.
After enough tests the failures become predictable. Nearly all of them are workmanship rather than design:
None of these are expensive to fix before plasterboard. All of them are expensive to fix afterwards. That is the entire argument for our pre-test checklist — we send it out ahead of the visit so the site team can walk the plot and close the obvious gaps while they are still reachable.
| Context | What testing is for |
|---|---|
| New build dwellings | Mandatory Part L compliance. Sampling regimes apply on multi-plot sites — we will advise on how many plots of each type need testing rather than testing more than you need. |
| Conversions & change of use | Where the works bring the building within scope of Part L, testing demonstrates the envelope performs as the design claimed. |
| Retrofit quality assurance | Not a regulatory requirement, but the only way to know whether the airtightness improvement you specified was actually delivered. Pre- and post-works testing on a sample of a programme is the cleanest evidence there is. |
| Ventilation design | A genuinely airtight dwelling needs designed ventilation. The test result is the number that tells you whether trickle ventilation is adequate or whether the design needs revisiting. |
It is worth being blunt about this, because it is where retrofit programmes cause harm. Making a building tighter without making its ventilation deliberate does not save energy — it moves moisture from somewhere it could escape to somewhere it condenses.
Every airtightness improvement should be accompanied by a ventilation strategy that has been designed rather than inherited. Where we are also acting as retrofit coordinator, that happens as a matter of course. Where we are testing for someone else, we will still tell you if the numbers suggest a ventilation problem is being built in.
Related: PAS 2035 retrofit coordination and portfolio EPCs.
Before You Ask
One Last Thing
Tell us how many plots, how many dwelling types and when you need sign-off. We will come back with the minimum number of tests that satisfies the requirement, a fixed price, and the pre-test checklist so the site team can get ahead of it.