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Future Homes Standard 2026: Notional Airtightness Target Changes
The Future Homes Standard moves the notional airtightness assumption from 5 to 4. Learn what this means for design, air testing and programme risk.
The Future Homes Standard retains the 8 m³/(h·m²) at 50 Pa limiting value for new dwellings, but tightens the notional dwelling used for compliance from 5 to 4 m³/(h·m²) at 50 Pa. This makes airtightness a more significant design and delivery issue for housebuilders.
Published in 2026, the standard comes into force in England on 24 March 2027, subject to project-specific transitional arrangements. A result of 5 may still comply, but it provides less headroom within the whole-dwelling calculation.
Key takeaways
- 4 m³/(h·m²) at 50 Pa is the Future Homes Standard’s notional airtightness assumption; 8 m³/(h·m²) at 50 Pa remains the limiting value.
- The notional value is the benchmark that shapes the compliance target. It is not a universal instruction that every dwelling must test at exactly 4.
- A dwelling that tests above 4 may need stronger performance elsewhere in the model, such as fabric, thermal bridging, glazing or services.
- The lowest-risk route is to design, detail, inspect and test the air barrier as a coordinated workstream—not to treat the final air test as a handover formality.
The number to design around is 4, not 8
Part L assesses a proposed dwelling against a notional dwelling of the same size and form. That reference dwelling has set assumptions for fabric, services, ventilation and airtightness. Under the Future Homes Standard, the notional airtightness assumption is 4 m³/(h·m²) at 50 Pa, compared with 5 under the Part L 2021 approach.
The 8 m³/(h·m²) figure has a different purpose. It is the limiting value: a dwelling cannot simply trade its way out of an air-test result above that threshold. For project teams, it is a legal backstop rather than a sensible design target. Building to the backstop leaves little room to manage variation in workmanship, services coordination or final test results.
Why a test result of 5 now needs a broader response
A dwelling that tests at 5 is not an automatic failure. The calculation looks at the dwelling as a whole. But because the reference home is now assumed to be tighter, the proposed home has more infiltration heat loss to offset. That can reduce compliance headroom and put more pressure on other elements of the design.
The right response depends on the dwelling type and the calculation. It may involve better continuity of insulation, more accurate thermal-bridge values, an improved glazing strategy, or fully coordinated heating and ventilation. The important point is that these choices are made while the design can still change—not after procurement or when the plot is close to completion.
Design the air barrier as a coordinated system
For architects, MEP consultants and contractors, airtightness should be a shared design responsibility. A robust approach is to:
- Design junctions, not just U-values. Thermal bridging and airtightness failures often appear at the same interfaces. Coordinate build-ups and use project-specific or accredited details where appropriate.
- Coordinate services before site installation. Penetrations for pipes, ducts, cables and extract terminals need ownership, access for sealing and a clear inspection route.
- Match airtightness to ventilation. A tighter envelope must be paired with a suitable, correctly installed and commissioned ventilation strategy. This is not a choice between airtightness and fresh air; both are required.
- Set an achievable project target. The design target, testing plan and site-quality checks should reflect the construction method and delivery team.
What this means for the development pipeline
The Future Homes Standard starts on 24 March 2027 in England. For eligible non-higher-risk projects, transitional provisions can apply where the relevant building-control notice or application was submitted before that date and work has commenced on the building before 24 March 2028. Confirm the route at project level, particularly on phased schemes.
That makes now the right time to review standard house types, thermal-bridge details, SAP strategy, ventilation approach and air-testing process together. The target is not simply a better test score. It is a repeatable specification that the design team and site team can deliver with confidence.
FAQs
Has the maximum air permeability changed?
No. The limiting value remains 8 m³/(h·m²) at 50 Pa. The key change is the tighter 4 m³/(h·m²) notional assumption used to set the compliance benchmark.
Must every new home test at 4 m³/(h·m²)?
No. A different result may still comply if the whole-dwelling calculation meets the relevant requirements. However, treating 4 as a practical design ambition can provide more resilience than relying on compensating measures elsewhere.
When should the SAP assessor and air-testing team be involved?
Bring them in at RIBA Stages 2–3, before fabric and services specifications are fixed. Keep them informed of design changes, then agree inspection and test points with the contractor before works start.
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