Part L 2026: what changes for air testing

The dates

24 March 2026: the 2026 editions of Approved Documents L and F were published.
24 March 2027: they take effect in England for building work that isn't higher-risk building work.
24 September 2027: they take effect for higher-risk building work.
Transition: there is a 12-month transitional period. Work for which an application or notice was made before 24 March 2027 can be built to the 2021 standard if work on that building starts before 24 March 2028. The arrangements apply building by building, not site-wide, so homes on the same site may be built to different standards. The detail is in MHCLG Circular Letter 01/2026.

What stays the same for air testing

Section 6 of Approved Document L, Volume 1 (2026 edition) carries over most of the 2021 rules:

An air pressure test should be carried out on every new dwelling (paragraph 6.5).
The limiting air permeability is still 8.0 m³/(h·m²) at 50 Pa, or 1.57 m³/(h·m²) at 4 Pa (Table 3.1).
Tests follow the approved methodology, CIBSE TM23, Testing Buildings for Air Leakage (paragraph 6.2), which covers both the fan pressurisation and low-pressure pulse methods.
The energy and emission rates calculated with the measured air permeability must not be worse than the targets (paragraph 6.6). If a home doesn't achieve the criteria, its air permeability should be improved and retested until it does (paragraph 6.7).
The results of all pressure tests, including failures, should be reported to building control (paragraph 6.8), and building control may accept a certificate from a registered tester (paragraph 6.4).

What changes

A tighter notional dwelling

Targets are set by a "notional dwelling" the same size and shape as the real one. Under the Future Homes Standard its air permeability is 4 m³/(h·m²) at 50 Pa, compared with 5 m³/(h·m²) in the 2021 notional dwelling. Designs will tend to assume lower air permeability, which leaves less room between the design value and a result that makes the as-built calculation miss its targets.

SAP 10.3 first, then the Home Energy Model

The notional dwelling is defined in Appendix R of the SAP 10.3 specification for the SAP method, and in the Future Homes Standard notional building tables for the Home Energy Model (HEM). SAP 10.3 is the only approved method at first; HEM was expected to follow, and on 8 June 2026 its launch was put back to "the coming months".

Calibration

The 2021 edition asks for evidence that test equipment was calibrated within the previous 12 months or the manufacturer's period, and recommends recalibration at least every 24 months. The 2026 edition asks for calibration in line with the manufacturer's guidance, CIBSE TM23 and at least once every 24 months (paragraph 6.3).

Ventilation testing alongside

The 2026 edition of Approved Document F says continuous mechanical ventilation should be measured with a powered flow hood (a vane anemometer may still be used for intermittent extract fans), and that the person commissioning should normally belong to an organisation that independently checks their competence, such as a competent person scheme. See ventilation testing.

What it means in practice

Check which standard each building is on. On sites that straddle the transition, homes can be on different editions.
Know the design air permeability for each house type before the test, so the tester and the site know what result the as-built calculation needs.
Build tight rather than test tight. With less margin, temporary sealing on the day is more tempting and less useful. Airtightness designed into the details lasts; tape applied for the test doesn't.
Plan for retests. Every plot is tested, and an analysis of six years of ATTMA lodgements to 2021 found about 15% of tests were retests after a fail.

This guide summarises published guidance; it isn't compliance advice. Your building control body decides whether a home complies, and your SAP assessor can tell you what each house type needs. Check the transitional arrangements for your own projects with building control.

Sources

Last checked: October 2026.