Part O routes: simplified method against TM59 modelling

Approved Document O gives you two ways to show a new home will not overheat, and the choice usually gets made by accident rather than on purpose. The simplified method is a set of geometric limits on glazing area and ventilation opening area. The dynamic modelling route runs the building through a thermal model against CIBSE TM59 criteria. Picking between the two Part O compliance routes early changes the elevations, so it belongs in the first design review, not in the building control submission.

The two routes and what each demonstrates

The simplified method asks whether the design falls inside a set of limits. If the glazing area is below the stated maximum for the orientation group and the ventilation free area is above the stated minimum, with the right cross-ventilation arrangement, the design is deemed to comply. No model, no consultant, no iteration.

The dynamic modelling route asks a different question. It predicts internal operative temperatures hour by hour for a design summer year and tests them against the TM59 criteria for predominantly naturally ventilated homes, with a separate test for bedrooms at night. It allows almost any design, and it costs money and time.

Both routes also require means of removing excess heat and both treat mechanical cooling as a last resort rather than a solution. That part is often missed.

Glazing and free area limits in the simplified method

The simplified method works off two tables. One caps the maximum glazing area, expressed as a proportion of floor area, by orientation group and by whether the dwelling is cross-ventilated. The other sets the minimum equivalent free area of openings, again as a proportion of floor area and again split by cross-ventilation. There is a further split between high-risk locations, which Approved Document O lists by area, and the rest of England.

Read those tables against your actual plans rather than an assumed layout, because three things trip schemes up repeatedly.

• A single-aspect flat is not cross-ventilated, whatever the corridor arrangement, so it falls into the tighter column.
• Equivalent free area is not the same as the opening light area. A top-hung window with a restrictor delivers a fraction of what the glazing suggests.
• Glazing on the south and west elevations is treated more harshly than north glazing, and a dual-aspect corner flat picks up the worse of its two orientations.

For a terrace of three-bedroom houses with modest windows and openable lights front and back, the simplified route is usually comfortable. For a single-aspect one-bedroom flat with a full-height window facing south-west, it almost never works.

When dynamic modelling becomes the only option

Four design decisions push a scheme into modelling.

1. Single-aspect dwellings, particularly those facing between south and west.
2. Glazing ratios above the simplified limits, which is to say most contemporary elevations.
3. Reliance on mechanical ventilation with heat recovery or on a mechanical cooling strategy.
4. A location where opening windows is not a realistic mitigation, usually because of noise or security.

Modelling is also the only route that lets you trade measures against each other. Shading, glazing specification, thermal mass and ventilation strategy all interact, and a model shows which lever actually moves the result. Fabric decisions made for the energy case do not automatically help here: a higher-performing wall holds heat in as well as out, which is why the U-values officers look at and the overheating result sometimes pull in opposite directions.

Commission the model early enough to change the elevation. A TM59 run delivered after planning permission, showing that the south-facing glazing has to shrink by a third, is an expensive piece of information.

Noise, security and restricted openings

Approved Document O accepts that an opening window is not always available. Where opening a window at night would expose occupants to excessive internal noise, the openings cannot be relied on for cooling, and the guidance points to accepted internal noise criteria for bedrooms. The same applies where restrictors are fitted for fall protection or where ground-floor windows cannot be left open for security.

This is where overheating and acoustics collide, and the two assessments are often commissioned separately by different people at different times. On a site beside a railway or a busy road, the honest answer is that the two have to be designed together. Acoustic ventilators, secure night-time openings and mechanical supply all appear in that conversation.

Officers in London will also ask about the cooling hierarchy in the London Plan 2021 policy SI 4, which sits alongside Part O rather than inside it. Reduce the heat gain first, then manage it passively, then ventilate, then cool mechanically if there is no alternative.

Mechanical cooling as the last step

Both compliance routes allow mechanical cooling, and both expect you to have exhausted the passive options first. A design that reaches TM59 compliance only with comfort cooling in every bedroom will be read as a failure of the architecture rather than a solution, particularly where a planning policy applies a cooling hierarchy.

There is a knock-on effect on the energy case. Cooling adds regulated energy, which worsens the carbon figure you report in an Energy Statement, and the notional building you are measured against is a frequent source of confusion in exactly this situation. If you are unclear how the baseline treats cooling, read the explanation of the notional building before the figures are drafted.

Where planning and building control diverge

Part O is a Building Regulations requirement, enforced by building control at the construction stage. Overheating in planning is a policy matter, driven by local plan policy, the London Plan in the capital and the design conversation with officers.

That split produces two practical problems. First, a scheme can win permission on elevations that will not pass Part O, because nobody tested them. Second, a council may ask for an Overheating Risk Assessment at application stage, which is a TM59 exercise in substance, while building control will want the same analysis again against the Approved Document criteria. Run one assessment that serves both and say in the covering text which criteria it tests.

What to submit with the application

Where a council asks for overheating evidence before determination, give it something that can be checked. A statement that the scheme will comply with Part O is not evidence.

For a simplified-method scheme, submit the glazing and free area calculations per dwelling type, the window schedule with equivalent free areas, and a plan marking which units are cross-ventilated and which are single-aspect. For a modelled scheme, submit the model report with the weather file used, the dwelling types tested and the worst-case results, plus the shading and ventilation assumptions that the result depends on.

Then make sure those assumptions survive into the conditions. External shading that disappears at technical design stage, or restrictors added later for fall protection, will quietly break a compliance case that looked settled at planning. Where carbon and overheating are both reported at outline stage, the sequencing problems described in carbon reporting in an outline application apply to the overheating work too.

Part O modelling is specialist work and councils expect a suitably qualified author, so professional review is strongly advised. For specialist technical reports, qualified professional input may be required.

This is general guidance, not legal advice. Check the current edition of Approved Document O and your local plan policy, and take advice from 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.