Reconciling Part O overheating with Part L in one design
Part L rewards a sealed, well insulated envelope and Part O penalises the solar gain that comes with it. The conflict is resolved at massing stage, not at technical design.
The Part O and Part L conflict is real and it is settled by one decision taken early: how much glass goes on which elevation. Approved Document L pushes towards a tight, well insulated envelope with efficient services. Approved Document O requires new residential buildings to limit unwanted solar gains in summer and to provide a means of removing excess heat. Both are calculated from the same model, and glazing area, orientation, g-value and openable area appear in both. Get the glazing ratio wrong and you will be redesigning after the planning application has been submitted.
Where the two requirements pull apart
The honest version of the tension is narrower than it is often described. Insulation and airtightness do not cause overheating on their own; large unshaded glazed areas, restricted ventilation and internal heat gains do. What a Part L compliant envelope does is retain the heat once it is in, so the same solar gain produces a higher and longer peak.
Three specific pinch points:
- South and west facing glazing gives useful winter gain in the Part L calculation and drives the summer peak in the Part O assessment
- Mechanical ventilation with heat recovery improves the Part L result on heat loss and adds fan energy, and it does very little for overheating unless it has a summer bypass and adequate purge provision
- Mechanical cooling would solve Part O and adds regulated energy to the Part L target, and Part O expects passive means to be exhausted first
Single aspect flats facing west are the reliable way to fail both, because there is no cross ventilation and the peak gain arrives late in the afternoon when the fabric is already warm.
Glazing area as the shared variable
Approved Document O offers two routes. The simplified method sets maximum glazing areas and minimum free areas for ventilation, varying by orientation, by whether the dwelling is cross ventilated and by whether the location is in the higher risk category the document identifies. The alternative is dynamic thermal modelling to the CIBSE overheating methodology.
The simplified method is fast and it is blunt, and on an urban scheme with limited openable area it often cannot be satisfied. Modelling gives more freedom and costs more, and it requires the design to be resolved enough to model.
Whichever route is used, glazing ratio is the variable that moves everything:
| Design move | Part O effect | Part L and design side effects |
|---|---|---|
| Reduce glazing area | Lower solar gain, easier compliance | Less daylight, possible conflict with daylight policy and sales expectations |
| Solar control glazing, lower g-value | Lower summer gain | Lower useful winter gain, reduced light transmission, cost |
| External shading, brise soleil, deep reveals | Substantial gain reduction with no energy penalty | Changes the elevation, so it is a planning matter |
| Larger openable areas | More purge ventilation | Constrained by noise, security, guarding and wind |
| Mechanical cooling | Removes excess heat | Adds regulated energy, expected only as a last resort |
Read that table the other way round and the answer is obvious: shading and orientation do the work, glazing area is the lever, and the mechanical options are expensive in both senses.
Shading measures and what they cost elsewhere
External shading is the most effective and the most visible measure, which is why it belongs in the planning drawings rather than in a later technical package.
Deep window reveals, projecting balconies, brise soleil, fixed fins and external shutters all reduce gain before it enters the building. Internal blinds do not, because the energy is already inside. Balconies are the most efficient option on a residential block, since they shade the floor below at no additional cost, though they shade it in winter too.
Three points design teams underestimate. Shading has to be modelled as built, including the depth of the reveal and any obstruction from adjacent blocks, or the assessment does not describe the building. Fixed shading sized for a south elevation performs poorly on a west elevation, where the sun is low, so the same detail cannot be repeated around the building. And a scheme that relies on user-operated shading has to show it will actually be operated.
Trees are not shading for the purposes of an assessment in the year of completion. They can be presented as an additional benefit with a stated maturity, no more.
Ventilation strategy and the energy penalty
Purge ventilation is what removes heat once it is in, and the free area available decides how much can be removed. Approved Document O sets minimum free areas, and on real elevations those areas are limited by restrictors, guarding, window type and the acoustic strategy.
Cross ventilation is worth designing for deliberately. A dual aspect dwelling with openable windows on two elevations removes heat at a rate a single aspect dwelling cannot match, and it usually costs less than every other measure. Where a scheme has many single aspect units, the overheating assessment is being asked to compensate for the plan.
The interaction with Approved Document F matters too. A dwelling with a very low design air permeability needs a continuous mechanical ventilation strategy, and the ventilation choice made for air quality then constrains the options available for summer heat removal. Take both decisions at once, alongside the Overheating Risk Assessment.
Noise and security, which narrow the options further
Approved Document O recognises that openable windows cannot be assumed on a noisy site. Where night-time noise would prevent windows being opened, the overheating assessment is carried out on the basis that they stay closed, which removes the simplest route to compliance and usually forces mechanical ventilation or cooling.
That puts a road, a railway or a night-time venue at the centre of the design. The acoustic assessment and the overheating assessment have to share assumptions about window condition, and a scheme where the acoustician assumes closed windows while the overheating model assumes open ones will be caught.
Security and safety narrow it further. Ground floor and accessible windows carry restrictors, guarding limits the openable area on upper floors, and wind on tall buildings restricts what can be opened at all. Each of those reduces free area, and each has to be reflected in the model rather than ignored.
Illustrative example: a four-storey block where the west-facing single aspect units are replaced with dual aspect units, glazing is reduced on the west elevation, 900 mm reveals are introduced on the south, and the road-facing flats use mechanical ventilation with a closed-window overheating assessment. Details here are illustrative only.
Sequencing the two calculations
Order the work so the expensive decisions are taken once:
- Test overheating risk at massing stage, on orientation, glazing ratio and aspect alone, before the layouts are fixed
- Set the glazing strategy by elevation, with shading drawn
- Choose the ventilation strategy, taking noise and air permeability together
- Run the Part L calculation with that envelope and ventilation system
- Run the full overheating assessment on the resolved design
- Reconcile, and record which measures serve both requirements
Teams that run Part L first and the overheating check last end up changing glazing after the planning application has been validated, which means a non-material amendment at best. The Energy Statement and the overheating work should be commissioned together.
What to commit to in the planning submission
Commit to performance, not to products. State the glazing ratio by elevation, the shading provided and its dimensions, the ventilation strategy, and the g-values required rather than a named glass. That leaves room for technical design without changing what was consented.
Say plainly where the two requirements conflict and how it was resolved. An officer reading a Sustainability Statement that acknowledges a reduced glazing ratio on the west elevation, with the daylight consequence assessed in the Daylight and Sunlight Assessment, will treat the scheme as considered. One that claims both requirements were met with no trade-off invites questions.
And keep the drawn shading in the application drawings, so a condition can secure it. Shading that exists only in a report gets value-engineered out.
Browse the PlanScope report library to see which energy and amenity documents a residential scheme is likely to need.
This is general guidance rather than legal advice. Check the position with your local planning authority, your building control body or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Do Part L and Part O actually contradict each other?
They pull in different directions on glazing and ventilation rather than contradicting. A well insulated, airtight envelope retains heat once it is inside, so the same solar gain produces a higher summer peak. The conflict is managed by reducing unwanted gain through shading and orientation rather than by weakening the fabric.
Which Part O compliance route should I use?
The simplified method is quick and sets maximum glazing areas and minimum free ventilation areas by orientation, cross ventilation and location risk. It often cannot be satisfied on constrained urban sites. Dynamic thermal modelling to the CIBSE methodology gives more design freedom, costs more and needs the design resolved enough to model.
Does internal shading count towards reducing overheating?
Internal blinds do very little, because the solar energy has already entered the building. External measures work: deep reveals, projecting balconies, brise soleil, fins and external shutters. Shading has to be modelled as built, including reveal depth and overshadowing from adjacent blocks, or the assessment does not describe the building.
What happens on a noisy site where windows cannot be opened at night?
Approved Document O expects the overheating assessment to be carried out with windows closed where night-time noise would prevent opening them. That usually forces mechanical ventilation or cooling. The acoustic assessment and the overheating assessment must share the same assumption about window condition.
When should the overheating assessment be done?
At massing stage, on orientation, glazing ratio and aspect, before layouts are fixed. Running the Part L calculation first and the overheating check last is how teams end up changing glazing after validation. Commission the energy and overheating work together so the glazing decision is taken once.
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.