Part O overheating and what it means for planning

Approved Document O has applied to new residential buildings in England since 15 June 2022, and it changes elevations. Requirement O1 asks a designer to limit unwanted solar gain in summer and to provide an adequate means of removing excess heat from the indoor environment. Both halves of that sentence land on the planning drawings: the first through glazing area, orientation and shading, and the second through openable windows, which is where Part O overheating collides with acoustic mitigation on any site next to a busy road. Deal with the conflict at planning stage or pay for it twice.

What Requirement O1 actually asks for

Part O applies to the erection of new dwellings, residential institutions, student accommodation and similar residential buildings. It is a building regulations requirement rather than a planning one, which is exactly why architects meet it too late.

Two compliance routes exist. The simplified method sets prescriptive limits, and the dynamic thermal modelling route demonstrates performance by calculation. Whichever route is taken, the design also has to deal with the practical constraints that come with opening a window: security on ground floor and accessible openings, protection from falling where the opening is large, and the effect of external air quality where the building sits in an air quality management area.

Conversions sit outside the requirement, because Part O bites on the erection of a building. That gap matters for anyone doing office to residential work, and councils have started closing it through local plan policy instead. Check the current scope of the Approved Document before assuming either way.

The simplified method and what it does to a facade

The simplified method works by limiting glazing area and requiring a minimum free area for ventilation, with different figures depending on whether the location is treated as high risk or moderate risk, and on whether the dwelling is cross-ventilated. Central London and parts of other major urban areas fall into the higher risk category.

The consequences show up on the elevation. Maximum glazing areas on the sunniest orientations are lower than most residential designers instinctively draw, so the full height south-facing window that sells the flat may simply be unavailable without shading. External shading, whether shutters, fins, deep reveals or overhanging balconies, is the usual answer, and every one of those is a planning matter because it changes the appearance of the building.

Single aspect north-facing flats pass more easily than single aspect south-facing ones. That is a plan-form consequence, and it interacts with the daylight and space standards a council will apply, so it needs testing before the layout is fixed rather than after.

The dynamic modelling route

Where the simplified method does not work, the alternative is dynamic thermal modelling to the CIBSE TM59 methodology for homes, or TM52 for non-domestic space. The model runs the building against a weather file and reports whether the criteria are met across the occupied hours.

TM59 has two criteria for naturally ventilated homes, one covering living areas, kitchens and bedrooms, and a separate bedroom criterion for the night period. Bedrooms are usually what fails. A room that performs acceptably during the day can still exceed the night criterion, because the fabric releases stored heat exactly when the occupant is trying to sleep.

Modelling gives design freedom in exchange for evidence and time. It also gives a defensible answer to a planning officer who asks whether the shading is doing anything, which the simplified route does not.

The night noise conflict

This is the part that catches schemes out. Removing excess heat almost always means opening windows at night, and Approved Document O will not let a design rely on openings where night-time noise makes them unusable. The Approved Document sets internal noise thresholds for that judgement, expressed as a level not to be exceeded across the night period together with a maximum level not to be exceeded more than a stated number of times a night, and where those are breached with windows open the openings cannot be counted.

Set that against the planning side. The Noise Impact Assessment submitted with the application will normally demonstrate acceptable internal levels by assuming windows closed, with acoustically treated background ventilation. That assumption is what the acoustician needs to satisfy the environmental health officer, and it is precisely the assumption that removes the overheating strategy.

Three ways out exist, in rising order of cost.

Orientation and layout, putting bedrooms on the quiet elevation and habitable rooms where cross-ventilation works.

Acoustically attenuated purge ventilation, which allows a high air flow with treatment in the path, so a window need not be opened for cooling.

Mechanical ventilation with cooling, which solves both problems and adds plant, energy demand and capital cost.

The first is free at concept stage and impossible at technical design stage. That is the whole argument for doing this early.

Where planning and building control collide

The regimes are separate and the timelines do not line up. Planning fixes the elevations, the glazing and the massing. Building control tests the thermal performance months later, against a design that planning has already frozen.

Two failure modes recur.

A permission granted with large unshaded glazing, followed by a Part O failure, followed by an application under section 96A or section 73 to add shading or reduce openings. That is a delay of weeks at best, and on a heritage-sensitive facade it can be far worse.

Or a scheme that passes Part O by adding comfort cooling, which then undermines the energy strategy submitted with the application. Where a local plan policy sets a carbon reduction target, retro-fitting cooling into the design can put the Energy Statement out of date and reopen the conversation with the council.

Evidence councils ask for at application stage

Requirements vary, and there is no national validation requirement for overheating evidence. In practice, three levels of expectation are common.

An Overheating Risk Assessment covering the modelled units, the assumptions on window opening and the interaction with the acoustic strategy answers all three. Where the council asks for TM59 output, councils normally expect a suitably qualified author, so professional review of any AI-assisted draft is strongly advised.

Sequencing so neither assessment is wasted

Run the acoustic and thermal work together, and run both before the elevations are fixed.

1. Take a noise survey early, and get the night-time levels at the facade rather than an estimate.
2. Test the layout for cross-ventilation and bedroom orientation while the plan is still fluid.
3. Choose the Part O route, and if modelling, model the worst case units first.
4. Agree the ventilation strategy with the acoustician before finalising window sizes.
5. Show the shading on the planning elevations, with a note explaining what it is doing.
6. Keep the modelling assumptions in the planning file so the building control submission does not contradict the planning one.

That last step matters more than it sounds. An application that says windows will be openable, followed by a building control submission that says they will not, is the kind of inconsistency that surfaces at the worst possible moment.

Follow reform news on the planning law timeline, and browse the report types by category to see where overheating and energy evidence fit into a full submission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. This is general guidance rather than legal advice, and the Approved Documents are amended periodically, so check the current published version before relying on any figure here.