Airtightness commitments made at planning stage
An air permeability figure in an energy statement is a promise that gets measured two years later by someone else. Pick it so it survives the test.
An airtightness commitment at planning stage is a number in an energy statement that someone will measure on site long after the permission is issued. Approved Document L Volume 1 (2021) sets a maximum air permeability of 8.0 m³/(h·m²) at 50 Pa for new dwellings, and the figure you declare in a planning energy model is almost always better than that, because the carbon improvement the policy requires depends on it.
Which creates the exposure. A design value of 3.0 gets the scheme through the planning arithmetic. An as-built result of 5.5 means the dwelling still complies with Part L, while the planning commitment the officer relied on is no longer true.
Why airtightness appears in an energy statement
An Energy Statement models the proposed dwellings against a notional Building Regulations baseline and reports the percentage improvement. Air permeability is one of the strongest single inputs in that model. Fabric U-values are bounded by what products exist, orientation is fixed by the site, and ventilation strategy is largely determined by the building type. Airtightness is the input the designer can move most freely on paper.
Policy drives the demand for improvement. London Plan policy SI 2 requires a percentage reduction on the regulations baseline through the Be Lean, Be Clean, Be Green hierarchy, with a stated minimum contribution from energy efficiency measures alone, and the Greater London Authority's energy assessment guidance sets the current figures. Many authorities outside London have adopted a similar structure in their local plans.
So the "be lean" stage is where air permeability earns its keep, and assessors reach for it when the fabric specification has run out of road.
Design targets against as built results
The gap between the two is well documented in the industry and it has a few consistent causes.
Design values are modelled on a continuous air barrier that is drawn but not always built. Trades that penetrate the barrier are sequenced after the trade that installed it. Site supervision of sealing is thin because it is invisible once the plasterboard goes on. And the person who chose the number was not the person who has to achieve it.
A realistic rule for a planning submission: declare a figure you would be comfortable writing into the contractor's specification. For masonry construction with wet plaster, something in the region of 3 to 5 is achievable with ordinary care. Below 3 needs a deliberate air barrier strategy, trained operatives and interim testing, and that has to be priced.
Timber frame and panellised systems can go tighter more reliably, because the air barrier is factory-applied and the junctions are detailed rather than improvised.
The testing regime and sample rates
Testing is carried out at the end of construction by a registered tester, in accordance with the approved methodology, and the result is a measured air permeability at 50 Pa. Both fan pressurisation and low-pressure pulse methods are recognised under the 2021 edition.
Part L 2021 moved away from the earlier sample-based approach for dwellings and expects new dwellings to be tested, with a penalty default value applied in the limited circumstances where a dwelling is not tested. The sampling rules and the default figure are set out in the Approved Document, and they have changed between editions, so take them from the current edition rather than from a precedent report.
The practical point for planning is simpler. The tested value is the number that exists in the real world, and it is held by building control rather than by the planning authority.
What happens when a test fails
A failed test is a building control matter first. The dwelling cannot be signed off, so the contractor seals and retests. Common remedies are sealing at service penetrations, loft hatches, soil vent pipe collars, window and door perimeters and the junction between the intermediate floor and the external wall.
The planning consequence depends entirely on how the commitment was secured. If the energy statement is simply an approved document with no condition attached to the air permeability figure, there is nothing to breach. If a condition requires the development to be constructed in accordance with the approved energy statement, and the statement states a design air permeability, an authority can take the view that the condition has not been complied with.
That is an uncomfortable position to be in with units sold and a warranty in place. The answer is to deal with it at the drafting stage rather than at the end.
Detailing that determines the outcome
Four junctions decide most results, and they are worth drawing at 1:5 before the planning energy model is signed off.
- The wall to intermediate floor junction, where joists or a slab edge interrupt the air barrier.
- The wall to roof junction at eaves and at any change of level.
- Service entry points, meaning the incoming water main, electricity, gas and any ductwork.
- Window and door reveals, where the air barrier has to be returned and bonded to the frame.
Specify who owns the air barrier on site. A single trade responsible for continuity, with a hold point and an interim test before the barrier is covered, changes outcomes more than any product choice. Interim testing at first fix is the single most effective intervention, because faults are still reachable.
Ventilation strategy follows from the target. A dwelling at 3.0 or tighter needs continuous mechanical ventilation designed properly, and the interaction with summer temperatures belongs in an Overheating Risk Assessment under the Part O methodology, for which councils expect a suitably qualified author and professional review is strongly advised.
Conditions that reference a tested value
Planning conditions have to meet the six tests in the Planning Practice Guidance on the use of conditions, and one of them is that a condition should not duplicate controls under other legislation. Air permeability is controlled by the Building Regulations. An authority that conditions a specific as-built air permeability is at risk of duplicating that regime.
Even so, conditions requiring compliance with an approved energy statement are commonplace, and they pull the figure in indirectly. Two drafting positions help.
Ask for the condition to be framed around the overall carbon or energy outcome rather than around a single input. A condition requiring the development to achieve the percentage improvement reported in the approved statement leaves the designer free to recover a shortfall elsewhere, through fabric, glazing or renewables.
Where a figure must appear, ask for a tolerance and a mechanism. Wording that allows an alternative specification achieving equivalent performance, approved in writing, is standard in well-drafted conditions and costs the authority nothing.
Keeping the planning and building control positions aligned
Treat the energy statement and the Part L submission as two views of one model, and keep them that way.
- Fix the design air permeability once, with the contractor's input if there is one appointed.
- Use the same figure in the planning energy model, the Part L calculation and the specification.
- Record in the statement that the figure is a design target to be demonstrated by testing under the Building Regulations.
- If the specification changes at tender, update the energy model before discharging the relevant condition rather than afterwards.
- Keep the test certificates on the planning file as well as the building control file.
Step three is the sentence that saves the argument. An energy statement that describes its air permeability input as a design target, verified through Building Regulations testing, has set the officer's expectation correctly. One that states a figure as an achieved performance has promised something nobody can deliver at planning stage.
Non-domestic schemes behave differently, because large envelopes and loading doors dominate the result, and we look at that in energy statements for warehouse and logistics schemes. Where the wider narrative needs pulling together, the Sustainability Statement is the place for it.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
What air permeability should I declare in a planning energy statement?
Declare a figure you would write into the contractor's specification. For masonry construction with wet plaster, 3 to 5 m³/(h·m²) at 50 Pa is achievable with ordinary care. Below 3 needs a deliberate air barrier strategy, trained operatives and interim testing, all of which has to be priced before the number is committed to.
Is 8.0 the Part L limit?
Approved Document L Volume 1 (2021) sets 8.0 m³/(h·m²) at 50 Pa as the worst acceptable air permeability for new dwellings. It is a limiting value rather than a target, and schemes relying on fabric performance for a planning carbon improvement will be modelling well below it. Check the current edition for the figures that apply.
Can a planning condition require a specific tested air permeability?
Authorities do impose such conditions, though they sit awkwardly with the guidance that conditions should not duplicate other regimes, and air permeability is controlled by the Building Regulations. Ask instead for a condition framed around the overall carbon improvement, or for wording allowing an alternative specification of equivalent performance to be approved in writing.
What if the as-built test is worse than the design figure?
Building control requires sealing and a retest before sign-off. Whether there is a planning problem depends on how the commitment was secured. A condition requiring construction in accordance with an approved energy statement that names a design figure can be treated as breached, so deal with the wording at the drafting stage.
Does tighter construction create an overheating problem?
Not by itself, but it changes the ventilation design. A dwelling at 3.0 or tighter needs properly designed continuous mechanical ventilation, and the summer temperature performance has to be assessed under the Part O methodology. Councils expect a suitably qualified author for that assessment and professional review is strongly advised.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.