Part O overheating and what it does to planning drawings

Part O of the Building Regulations requires new residential buildings in England to limit unwanted solar gain in summer and to provide an adequate means of removing excess heat. Approved Document O gives two compliance routes: a simplified method that caps glazing areas and sets minimum free areas for openings, and dynamic thermal modelling, normally carried out to CIBSE TM59 for homes. Either route changes window sizes, shading and the way flats are ventilated, which is why it belongs in the planning drawings rather than in a post-permission Building Regulations exercise.

Architects who leave it until the technical design stage end up back at the council. A scheme approved with full height glazing on a south facing single aspect flat is not going to pass, and shrinking the windows afterwards is a material change to the approved elevations.

What Part O covers and what it does not

Part O applies to new residential buildings, including dwellings, residential institutions and student accommodation. It does not apply to a material change of use, so an office to residential conversion under Class MA sits outside it, although overheating remains a planning consideration and a design risk in its own right.

Two requirements sit behind the Approved Document. Limit unwanted solar gains in summer. And provide an adequate means of removing heat from the indoor environment. Read them as a pair, because a design can satisfy one and fail the other.

Verify the current version of the Approved Document on gov.uk before you rely on any figure. The document has been amended since it first came into force in June 2022, and transitional arrangements attach to specific dates.

The simplified method against dynamic modelling

The simplified method sets maximum glazing areas as a proportion of floor area and minimum openable free areas, varying with whether the dwelling is cross-ventilated and whether it sits in a location the document identifies as higher risk. Parts of London are treated as higher risk.

It is quick and it needs no specialist software. It is also blunt. A single aspect south facing flat with a generous window and no cross-ventilation will fail it, and the fix is either less glazing or a different plan.

Dynamic thermal modelling to CIBSE TM59 tests the actual design against defined weather data and occupancy profiles, using the criteria that the methodology sets for living spaces and bedrooms. It gives the design team room to trade a larger window against external shading, a different orientation or a better ventilation strategy. It costs more and it needs a competent modeller.

For anything over a handful of flats, model it. The simplified method on a single aspect block usually produces windows the planning officer will call mean and the sales team will hate.

Where overheating and noise pull in opposite directions

This is the conflict that produces late redesigns. The simplified method assumes windows can be opened. On a site next to a main road, a railway or a night time economy frontage, the acoustic assessment will conclude the opposite, because opening the window lets in sound that breaches the internal levels in BS 8233:2014.

Approved Document O deals with the collision by giving indicative night time noise levels above which windows are assumed to be shut, and by requiring an alternative means of removing heat where that applies. Check the figures in the current Approved Document rather than a summary, because they are specific and they matter.

Where windows are assumed shut, the practical answers are mechanical ventilation with heat recovery sized for summer bypass, acoustically treated background ventilators, or, as a last resort, active cooling. Each has a planning consequence: plant space, louvres on an elevation, a condenser deck on the roof or a slightly deeper floor to ceiling zone.

Get the acoustician and the overheating modeller in the same conversation early. They are usually working to different assumptions about whether the window opens, and nobody notices until the reports are written.

Effect on windows, balconies and shading

The design moves that come out of Part O are visible ones.

• Reduced glazing ratios, often achieved with a deeper pier between openings rather than a smaller window.
• External shading: brise soleil, fixed fins, projecting reveals, shutters and, on some schemes, balconies positioned to shade the storey below.
• Solar control glazing, which changes the reflectance and colour of the elevation and can attract comment in a sensitive setting.
• Openable areas expressed as specific window types, since a top hung vent gives a different free area from a side hung casement.
• Inset balconies that reduce solar gain but also reduce daylight, which then pulls the daylight and sunlight assessment in the other direction.

Balconies are the recurring argument. A projecting balcony shades the flat below and provides useful outdoor space. An inset balcony shades its own room and reduces its daylight. Whichever you choose, the elevation reads differently, and changing it after permission is a section 73 application rather than a detail.

Coordinating with the planning submission

Councils in England vary in what they ask for at application stage. Some request an overheating statement as a validation item, some fold it into the energy or sustainability statement, and London boroughs apply the London Plan cooling hierarchy with its own expectations for major development. Check the local validation list before you assume nothing is needed.

A workable sequence:

1. Fix orientation and the plan type at concept stage, because single aspect units are where overheating risk concentrates.
2. Run an early simplified method check on the worst units to see whether modelling is needed.
3. Commission the acoustic survey and the overheating model together, with a shared assumption about window opening.
4. Reflect the shading, balcony and glazing decisions in the planning elevations.
5. Explain the strategy in the sustainability or energy statement submitted with the application.

If the response to a failed overheating model is a second, overlapping application rather than a variation, read the Hillside judgment on overlapping permissions first, because the two permissions may not be capable of being implemented together. Follow changes to the Approved Documents through the planning law timeline rather than relying on an older summary.

Avoiding the late redesign

The expensive failure runs like this. Permission granted on drawings with large windows. Building Regulations stage arrives. The overheating model fails on the south facing single aspect units. Glazing is cut by a fifth, and the elevation no longer matches the approved drawings, so a section 73 application follows with a fresh consultation period and a new decision to wait for.

Avoid it by testing the worst unit before the planning submission, not after. One afternoon of modelling at concept stage buys more certainty than any other hour spent on the scheme.

Draft the sustainability and energy documents alongside the design using the report types, and check PlanScope pricing if you want to edit and version drafts across a project library.

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, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.