Most of the Future Homes Standard conversation has been about Part L — the notional dwelling, heat pumps, PV, U-values. But the change that’s most likely to bite on site sits in the other document. Approved Document F Volume 1, 2026 edition introduces a formal split between “less airtight” and “highly airtight” dwellings, and that single definition decides what ventilation system you’re allowed to use.
The definition
AD F 2026 defines it in the glossary. A highly airtight dwelling is one that achieves one or both of the following:
- a design air permeability lower than 5 m³/(h·m²) at 50 Pa, or
- an as-built air permeability lower than 3 m³/(h·m²) at 50 Pa.
Everything else is a “less airtight dwelling”. That’s the whole test — two numbers.
Now put it next to Part L. The FHS notional dwelling under the SAP 10.3 route assumes an air permeability of 4 m³/(h·m²), against a legal backstop that stays at 8. So a dwelling designed to match the notional specification is, by definition, a highly airtight dwelling under Part F. The two documents are deliberately joined at the hip.
Why the label matters
Table 1.6 of AD F 2026 sets out which ventilation systems the guidance covers for which dwellings:
- Natural ventilation — background ventilators plus intermittent extract fans — is listed as suitable for less airtight dwellings only.
- Continuous mechanical extract ventilation (centralised c-MEV or decentralised d-MEV) covers all dwellings.
- Mechanical ventilation with heat recovery covers all dwellings.
In other words, once a home is highly airtight, trickle vents and a humidistat fan in the bathroom stop being a route to compliance. You need continuous mechanical ventilation, or a specialist design.
That’s not surprising in itself — squeeze out uncontrolled infiltration and something has to replace it. What’s new is that the guidance now says so explicitly, with a number attached.
The bit that catches people out
Here’s the paragraph worth reading twice. AD F 2026, paragraph 1.46: where a dwelling has natural ventilation and a measured air permeability that differs from the design air permeability, so that it becomes defined as a highly airtight dwelling, one of the following applies:
- expert advice should be sought, or
- a continuous mechanical extract ventilation system should be installed.
Read that as a site scenario. A plot is designed at, say, 5.5 m³/(h·m²) with a natural ventilation strategy — perfectly reasonable, less airtight dwelling, trickle vents and intermittent extract. The build team does a genuinely good job on the tapes and seals. The as-built test comes back at 2.8. That plot is now a highly airtight dwelling, and the ventilation system that’s already installed is no longer the one the guidance supports.
Building tighter than you designed used to be a small bonus on the SAP result. Under AD F 2026 it can also be a compliance problem — and it lands right at the end of the programme, after first fix.
The practical answer is to decide early. If there’s any prospect of the as-built result coming in under 3, design for continuous mechanical ventilation from the start rather than hoping the test lands in the right band.
What continuous MEV actually asks for
If you do land in continuous extract territory, a few of the numbers are worth having to hand:
- Whole-dwelling ventilation rate — the higher of 0.3 l/s per m² of internal floor area (all floors) and the bedroom-based figure in Table 1.3: 19 l/s for one bedroom, 25 for two, 31 for three, 37 for four, 43 for five, adding 6 l/s per bedroom beyond that. If the dwelling has only one habitable room, 13 l/s.
- Total continuous extract should be at least the whole-dwelling ventilation rate.
- Background ventilators are still required with c-MEV and d-MEV. They should not be in wet rooms, should provide a minimum equivalent area of 4,000 mm² per habitable room, and the total number should equal the number of bedrooms plus two — so a one-bed needs three, a two-bed needs four. They’re meant to be left open.
- Internal doors need a free area equivalent to a 10 mm undercut in a 760 mm door, measured above the finished floor.
MVHR is the exception on background ventilators: because the unit supplies filtered air to habitable rooms directly, they aren’t required — but you then need room for two sets of ductwork, and AD L 2026 wants at least 73% heat recovery efficiency plus a summer bypass.
When this applies
The amended Approved Documents L and F come into force on 24 March 2027 for most new building work, and 24 September 2027 for higher-risk building work, with transitional protection running to 24 March 2028 for plots that qualify. If you’re designing now for a 2027 start, you’re designing to these rules.
What to do about it
Three things worth doing before the deadline gets close:
- Pick the ventilation strategy from the target air permeability, not the other way round. If the design figure is under 5, natural ventilation is off the table from day one.
- Stress-test the “we built it too well” scenario. Ask what happens if the as-built test comes back under 3, and make sure the answer isn’t “retrofit a ducted system into a finished house”.
- Get the junctions right anyway. Airtightness and thermal bridging live in the same places — junctions, penetrations, service voids. Tighter fabric makes both your psi-values and your air test result more important, and neither is something you want to discover late.
ΨMonkey calculates bespoke PSI values for your junctions to BR 497 and BRE IP 1/06 conventions, so the details you’re actually building show up in your SAP model instead of the worst case the software assumes.
And if you’ve got a plot where the air test has come in tighter than the design figure and you’re not sure where that leaves the ventilation strategy, get in touch — it’s a conversation worth having before first fix, not after.
Sources: Approved Document F Volume 1, 2026 edition (PDF), GOV.UK — glossary definition of “highly airtight dwellings”, Tables 1.3 and 1.6, and paragraphs 1.20, 1.21, 1.46 and 1.64 · CIBSE Journal CPD Module 266: Ventilation design under the Future Homes Standard, July 2026 · The Building Regulations etc. (Amendment) (England) Regulations 2026 (SI 2026/335), legislation.gov.uk