What Part L now requires on every new dwelling
Since the 2021 edition of Approved Document Part L came into force in June 2022, airtightness testing on new homes changed in one important way: every single dwelling has to be tested. The old approach, where a builder tested a sample of each house type and applied the result to the rest, is no longer allowed for new dwellings. Every plot needs its own on-site test and its own certificate before building control will sign it off.
The test measures air permeability (how much air leaks through the building fabric), and the number it produces goes straight back into your as-built SAP calculation. That's the part people underestimate: the air test isn't a box-tick at the end, it's the figure that decides whether the finished home actually meets the emission and energy targets it was designed to.
The Part L air test, in plain numbers
- Every dwelling is tested. Sample testing is no longer permitted for new homes: one test, one certificate, per plot.
- 8.0 m³/(h·m²) at 50 Pa is the absolute cap. Go above it and the dwelling fails outright.
- But 8 is rarely your real target. The SAP 10 notional dwelling assumes 5.0 m³/(h·m²), so most designs need to land around 5, or tighter, to pass on emissions and primary energy.
- The result feeds your as-built SAP. Miss your design figure and the recalculation can push the plot out of compliance.
- Testing follows ATTMA standards. Both Pulse and blower-door methods are accepted for compliance.
Whether you're building one home or a hundred
The regulation is the same, but the pressure it puts you under is not. Tell us which one you are and we'll shape the visit around it.
Self-builders & one-off homes
You've got one shot at a clean sign-off, and the air test is one of the last things standing between you and moving in. We walk you through what a good result looks like, when to book, and what to seal beforehand, so it passes first time without drama.
- Plain-English guidance, not jargon
- Advice on when in the build to test
- Result on the day, certificate within 24h
Developers & house-builders
Every plot now has to be tested, which means the air test can quietly become the thing that holds up handover and sales. We work to your programme, test plots in batches to keep cost per test down, and turn certificates around fast so building control isn't waiting on us.
- Multi-plot visits, discounted per test
- Booked around your completion dates
- Fast certificates so handovers don't stall
Why we test with Pulse
Most testers still arrive with a blower door: a big calibrated fan sealed into a doorway that pressurises the whole house to 50 Pa. It works, but it's slow, intrusive, and pushes the building to pressures it never actually sees in real life.
The Pulse method takes a different approach. It releases a short, controlled burst of air and measures how the home leaks at around 4 Pa, much closer to the pressure a building experiences on a normal breezy day. That single difference makes the whole test quicker, gentler and more representative.
| What we compare | Pulse (our method) | Traditional blower door |
|---|---|---|
| Test pressure | ~4 Pa, close to real-world, in-use conditions | 50 Pa, well above everyday conditions |
| Time on site | Under 15 minutes for the test itself | Longer: setup, sealing, multiple ramps |
| Disruption | No fan sealed into a doorway; nothing taped up | Fan mounted in a doorway; openings sealed |
| Stress on the fabric | None abnormal, no pressurise/depressurise cycle | Higher loads on the building envelope |
| Result | Shown on screen immediately | Calculated after the test run |
| Part L compliance | Accepted | Accepted |
To be clear: a blower door is a perfectly valid compliance test. We use Pulse because, for a busy new-build programme, faster and less intrusive means fewer delays, and a 4 Pa reading is a fairer picture of how the finished home will really perform.
How the test works, step by step
1. Book it into your programme
The ideal time to test is once the dwelling is sealed (external doors and windows in, service penetrations sealed, trickle vents fitted), but with enough time left to fix anything and retest before completion. Give us the site postcode and your target date and we'll slot it in.
2. We test on site
The Pulse equipment is carried in by one person, set up in minutes, and the test itself takes under 15 minutes. You see the air permeability figure on screen straight away, so there's no waiting to find out where you stand.
3. You get your certificate
We lodge your ATTMA certificate, the document building control accepts for Part L, within 24 hours, or the same day if you need it. The measured figure is ready to drop into your as-built SAP.
If a plot doesn't pass first time
It happens, and on its own it is not a Part L failure. A test that comes in above target simply tells us where the home is losing air. From there it's straightforward: we pinpoint the leaks, you carry out the remedial sealing, and we come back and retest.
Additional tests on the same plot are discounted. You're not paying full price twice because a service penetration needed sealing. And where a figure is close, tightening it can often be offset against other elements in the SAP calculation to reach compliance without endless re-sealing. The goal is a signed-off plot, with the least disruption to your programme.
What's included, and how the pricing works
Every test includes the on-site Pulse measurement, an immediate result, and an ATTMA certificate lodged for building control. What we don't do is charge every plot as if it's a standalone job.
- More tests at one site, lower cost per test. When we can test several plots in a single visit, the travel and setup are shared, so the price per plot comes down.
- Discounted retests. If a plot needs sealing and a second test, that retest is charged at a reduced rate.
- No surprises. You get a clear quote for your specific site before we book anything.
Tell us how many plots you've got and when they're completing, and we'll put a fixed price in front of you.
Get a quote for your site →Why builders across the North West use Thermova
- ATTMA-registered testing. Tests are carried out to ATTMA standards and your certificate is lodged on the ATTMA database, the record building control accepts for Part L.
- Local, so we turn up when you need us. Lancashire and the Fylde Coast are our doorstep, and we cover the wider North West for developments.
- Fast certificates. Within 24 hours as standard, same day on request, because a slow certificate holds up a handover.
- We test as we build. Thermova installs heat pumps, solar and battery systems, so we understand the whole low-energy home, not just the fan reading.
Frequently asked questions
Is airtightness testing mandatory on a new build?
What air test result do I need to pass Part L?
How long does an airtightness test take?
What is the difference between Pulse and a blower door test?
When in the build should the air test be done?
What happens if a plot fails the air test?
Do you cover my area?
Explore further
Next step
Book your Part L air test before the date slips
Every day a plot sits untested is a day it can't complete. Send us the site postcode, the number of plots and your target dates, and we'll come back with availability and a fixed price.