Part O overheating against CIBSE TM59 in planning
The decisions that decide Part O compliance are made when the elevations are drawn. Here are the two compliance routes, the modelling criteria and the noise conflict on urban sites.
Part O is a Building Regulations requirement, not a planning one, and that split is the source of most of the trouble. The decisions that determine whether a residential scheme can comply with Part O are made at planning stage: orientation, glazing proportion, aspect, shading, balcony depth and whether windows can be opened at night. By the time the building control application is prepared, those decisions are fixed in a permission.
Approved Document O has applied to new residential buildings in England since June 2022. It offers two routes to compliance, and the choice between them should be made at concept design rather than at technical design.
The two compliance routes
The simplified method sets limits on the maximum area of glazing and a minimum free area for removing excess heat, varying by whether the location is high risk or moderate risk and whether the dwelling is cross-ventilated. It is quick to check and it is unforgiving on the kind of scheme developers like: single aspect flats with generous glazing facing south or west.
The dynamic thermal modelling route uses the CIBSE methodology for overheating in homes, with the criteria applied to living rooms, kitchens and bedrooms. Two tests have to be passed:
- For occupied rooms, the number of hours during which the difference between operative temperature and the comfort temperature is one degree or more must not exceed 3 per cent of occupied hours between May and September.
- For bedrooms, operative temperature must not exceed 26 degrees for more than 1 per cent of annual hours between 10pm and 7am.
Modelling uses the design summer year weather files appropriate to the location, and the file chosen makes a real difference to the result. State which weather file and which emissions scenario the model used, because a reviewer cannot interpret the output without it.
Part O also covers matters beyond temperature: guarding where windows open at low level, protection against entrapment, security, external noise at night and outdoor air pollution. Those provisions are what make the noise conflict a design problem rather than a footnote.
Where planning and Part O collide
Five design decisions are effectively locked by a planning permission, and all five drive overheating.
| Decision | Effect on overheating |
|---|---|
| Orientation and aspect | Single aspect west-facing flats are the hardest to make compliant |
| Glazing proportion and g-value | Solar gain scales directly with glazed area and transmittance |
| External shading | Brise soleil, reveals, balconies and shutters all change the elevation |
| Openable area and window type | Free area for purge ventilation depends on the opening type and restrictors |
| Night-time ventilation | Depends on whether windows can be opened at night at all |
Changing any of them after permission means a non-material amendment, a section 73 application or a fresh application, with the delay and consultation that implies. That is why the overheating strategy belongs in the design at the point the elevations are drawn.
An Overheating Risk Assessment run at concept design tells you which flats are marginal while the plan can still change. Run at technical design, it tells you which flats are a problem.
When an assessment is required at planning stage
Nationally there is no requirement to submit an overheating assessment with a planning application. Two things change that.
In London, the London Plan requires major development to demonstrate the cooling hierarchy, reducing heat gain through the design before adding active cooling, and boroughs ask for the modelling at application stage. That is a London requirement and should be treated as such.
Outside London, some authorities have adopted policies on overheating or climate resilience that ask for the same evidence, usually on major residential schemes. Check the adopted local plan and the validation checklist before assuming the assessment is a building control matter alone.
Where an assessment is submitted at planning stage, be explicit that it is a design stage model based on assumed specifications, and that the Part O compliance case will be made at building control. That framing avoids being held to a set of assumptions that will change.
The noise conflict, and how to resolve it
The hardest cases are urban sites where the acoustic strategy assumes closed windows and the overheating strategy assumes open ones.
Internal noise levels for sleeping are conventionally assessed against the British Standard guidance on sound insulation for buildings, and where an open window would exceed those levels, the acoustic report proposes closed windows with mechanical ventilation. Part O then removes night-time openable ventilation from the overheating strategy, and the same flat has to lose heat without opening a window.
The resolutions are limited and they all cost something:
- Acoustically attenuated ventilators sized for both the noise attenuation and the free area needed for cooling.
- Winter garden or acoustic balcony arrangements that allow a window to be opened behind a screened zone.
- Mechanical ventilation with heat recovery, with a summer bypass, which helps with air quality but does little for peak temperature on its own.
- Active cooling, which the London cooling hierarchy treats as a last resort and which carries an energy and carbon penalty.
- Design change: dual aspect, relocating bedrooms to the quiet elevation, or reducing glazing on the exposed facade.
Number five usually costs least and is the least popular. Deal with it early, because a Noise Impact Assessment and an overheating model produced by two consultants who have not spoken to each other will contradict each other, and a reviewer will notice.
Where it sits with the other documents
The Energy Statement deals with regulated energy, the energy hierarchy and carbon reduction against the target emission rate. Overheating is a separate calculation with separate criteria, and a scheme can perform well on energy and badly on overheating. A highly insulated, airtight envelope with large glazing is exactly that combination.
Daylight is the other tension. Reducing glazing helps overheating and hurts daylight, and both are assessed. Where a scheme is tight on both, the resolution is usually shading rather than glass area, because external shading reduces solar gain while preserving the view of sky that daylight calculations depend on.
The policy narrative that ties them together belongs in the Sustainability Statement, with the numbers cross-referenced rather than restated.
Modelling assumptions that get challenged
Reviewers ask about the same five inputs every time.
Internal gains. Occupancy profiles, equipment and lighting gains should follow the published methodology rather than optimistic assumptions.
Window opening behaviour. The model assumes occupants open windows when it is warm, and that assumption fails where restrictors, security concerns or noise mean they do not.
Blinds and curtains. Internal blinds do far less than external shading, and a model that credits internal blinds heavily is a weak model.
Corridor temperatures. Communal corridors with uninsulated heating distribution can run hot, and adjacent flats gain heat through the party wall.
Weather file. The choice of design summer year and emissions scenario, and whether a future climate scenario has been tested.
Illustrative example. A block of single aspect flats facing south west passes the modelling with night-time window opening assumed throughout. The acoustic report for the same scheme requires closed windows on that elevation because of a nearby main road. Reconciling the two produces attenuated ventilators, deeper reveals and a reduction in glazing on the upper floors. This example is illustrative only.
What to do at each stage
At concept design, test the simplified method, identify the flats that cannot pass it, and model those early. At planning stage, submit the assessment where policy requires it and describe the assumptions honestly. At technical design, rerun the model against the specified glazing, shading and ventilation, and check that nothing in the value engineering has removed the shading that made it work.
Councils usually expect a suitably qualified author for thermal modelling, and professional review is strongly advised before submission.
This is general guidance, not legal advice. Check the adopted policy and validation requirements with your local planning authority, and the current text of the Approved Document, before relying on it.
Browse the PlanScope report types to see which energy and comfort documents your scheme is likely to need.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Is Part O a planning requirement?
No. Part O is a Building Regulations requirement applying to new residential buildings in England since June 2022. It matters at planning stage because orientation, aspect, glazing proportion, external shading and whether windows can be opened at night are all fixed by the permission, and changing them later needs an amendment application.
What are the two Part O compliance routes?
The simplified method sets limits on maximum glazing area and a minimum free area for removing excess heat, varying by risk location and whether the dwelling is cross-ventilated. The alternative is dynamic thermal modelling using the CIBSE methodology for overheating in homes, which is the route most urban apartment schemes have to take.
What criteria does the modelling route apply?
Two tests. For occupied rooms, hours where operative temperature exceeds the comfort temperature by one degree or more must not exceed 3 per cent of occupied hours from May to September. For bedrooms, operative temperature must not exceed 26 degrees for more than 1 per cent of annual hours between 10pm and 7am.
How do you resolve the conflict between noise and overheating?
Options are acoustically attenuated ventilators sized for both attenuation and cooling airflow, winter gardens or acoustic balconies that allow openable windows behind a screen, mechanical ventilation with a summer bypass, active cooling as a last resort, and design change such as dual aspect or reduced glazing on the exposed facade.
Which modelling assumptions get challenged most?
Internal gains and occupancy profiles, assumed window opening behaviour where restrictors or noise prevent it, over-reliance on internal blinds instead of external shading, heat gain from hot communal corridors through party walls, and the choice of design summer year weather file and emissions scenario.
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.