Overheating under Part O: the two assessment routes

Approved Document O gives you two routes to demonstrate a Part O overheating assessment: the simplified method, which caps glazing area and sets a minimum openable area by orientation and ventilation type, or dynamic thermal modelling against the CIBSE TM59 criteria. The simplified method is faster and free. Dynamic modelling costs money and buys the glazing the architect actually drew. Choosing between them is a design decision rather than a compliance formality, and it needs to happen before the planning elevations are fixed.

Part O applies to new residential buildings, which covers dwellings, residential institutions and student accommodation. It does not apply to a material change of use, so an office converted to flats under Class MA falls outside it, which is one of the reasons converted schemes sometimes perform worse than new build on summer comfort.

What the requirement actually asks for

Requirement O1 asks that reasonable provision be made to limit unwanted solar gains in summer and to provide an adequate means of removing excess heat from the indoor environment. Two limbs, and both have to be satisfied. Requirement O2 asks for information about the overheating strategy to be given to the owner.

That framing explains why shading and ventilation are treated as a pair. A scheme with modest glazing and no way of getting heat out fails, and so does a scheme with generous openings and unshaded west-facing glass.

The simplified method and where it runs out

The simplified method works from tables. It divides England into moderate risk and high risk locations, with the high risk locations being specified parts of London, and it treats cross-ventilated dwellings differently from single-aspect ones. For each combination it gives a maximum glazing area and a minimum free area of openings, both expressed against floor area.

It is quick, it needs no modelling fee, and on a cross-ventilated house in a moderate risk location with ordinary window sizes it usually passes without any special measures. Its limits are sharp.

• Single-aspect flats in high risk locations have very little headroom, particularly where the aspect is south or west.
• The method assumes windows can be opened, so it cannot accommodate a scheme where noise or security prevents that.
• Large glazed areas, full-height windows and glazed corners will generally fail it.
• Mechanical cooling cannot be used to buy compliance under the simplified route.

If any of those apply, the choice has already been made for you.

Dynamic thermal modelling and what it constrains

The modelling route runs a thermal model of the dwelling against the TM59 criteria for homes: a limit on the hours during which the operative temperature exceeds the adaptive comfort threshold across the summer occupied period, and a separate night-time criterion for bedrooms based on hours above 26 degrees between 10pm and 7am. Both have to pass.

Approved Document O constrains the modelling in ways that matter to the result, and this is where inexperienced modellers get an answer that later unravels.

• Weather data is specified, so a modeller cannot pick a mild year.
• Curtains and blinds cannot be assumed to provide solar shading, because the occupant may not use them.
• Openings can only be assumed open where they are safe, which means fall protection and security are design constraints on the ventilation strategy rather than afterthoughts.
• Where night-time noise would require windows to be shut, the model has to reflect closed windows, which usually forces mechanical ventilation with a purge capability.

The noise interaction is the one that most often changes the elevations. Approved Document O sets internal noise levels above which windows should be treated as closed at night, drawing on the established night-time criteria for bedrooms. On an urban site beside a busy road, that turns a naturally ventilated scheme into a mechanically ventilated one, and the acoustic evidence has to be produced to support it. In practice that means the Noise Impact Assessment and the overheating model have to be written from the same facade level survey rather than from separate assumptions.

Design consequences that reach the planning drawings

Part O is building control, not planning. But the measures it drives are nearly all visible, and a scheme consented on one set of elevations and then redesigned for Part O comes back as an amendment.

Three of those five change the external appearance. That is the argument for running the overheating strategy before the planning submission rather than after, and it is the same argument that applies to fire and structural coordination, where the sequencing lesson is set out in the note on the Building Safety Regulator gateways for planning teams.

Local policy adds a second layer in some areas. The London Plan requires major development to follow a cooling hierarchy and to submit evidence on overheating, so in London an Overheating Risk Assessment is a planning submission document in its own right and not merely a building control exercise.

How conditions and Part O interact

Planning conditions cannot require compliance with the Building Regulations, because that duplicates a separate statutory regime. What they can do is secure the measures the overheating strategy relies on, where those measures are material to the planning judgment.

1. Approve external shading details, including depth and material, before the relevant part of the work starts.
2. Approve the position, size and appearance of ventilation louvres and plant enclosures.
3. Require the scheme to be built in accordance with the approved elevations, which indirectly protects the shading.
4. Where mechanical ventilation is the strategy, secure the plant space in the approved drawings so it cannot be lost to a later layout change.

Avoid a condition that requires a TM59 report to be submitted to the planning authority. The authority has no remit to approve it, the applicant gains nothing, and discharging it wastes a fee. Where local policy does require the evidence, ask for it with the application instead.

What an overheating report should contain

Whichever route is used, the report needs to be readable by someone who did not build the model. State the route, the risk category of the location, the ventilation type of each assessed dwelling, the sample selection and why it is representative, the assumptions on shading and openings, the noise position, and the results against each criterion for each assessed room.

Sample selection deserves particular care. A block assessed on its best-performing flats will pass and then fail on site. Pick the worst cases by aspect, floor level and glazing ratio, and say why they are the worst cases.

Where energy and overheating are assessed together, keep the Energy Statement and the overheating work aligned on fabric assumptions, because a fabric specification that helps winter heat loss can worsen summer performance. Follow reform to this area through the planning law timeline, since the Approved Documents are revised on their own cycle.

This is general guidance, not 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.