TM59 modelling or the Part O simplified method
The simplified method in Approved Document O sets fixed glazing and free area limits. TM59 modelling accepts any geometry but has to evidence everything. Here is how to choose between them at concept stage.
Choosing between TM59 or the Part O simplified method is a decision about how much design freedom you are willing to trade for certainty. The simplified method in Approved Document O gives fixed limits on glazing area and on the free area of openings, and if the design sits inside them the overheating requirement is satisfied without modelling. Dynamic thermal modelling to CIBSE TM59 costs money and time, but it accepts any geometry and lets shading, thermal mass and ventilation strategy do the work.
For most housing schemes the answer is settled at concept stage, whether anyone says so or not. Glazing ratio decides it.
What each route asks for
Approved Document O came into force in June 2022 and applies to new residential buildings, including dwellings, residential institutions and student accommodation. It sets two objectives: limit unwanted solar gain in summer, and provide a means of removing excess heat.
The simplified method meets both through prescription. Maximum glazing areas are set by orientation grouping, and minimum free areas for openings are set by whether the dwelling is cross-ventilated. The limits tighten in the high risk locations listed in the Approved Document, which cover parts of London and a small number of other urban areas.
The dynamic modelling route meets them through performance. A thermal model of the actual dwelling is run against CIBSE design summer year weather data and tested against the TM59 criteria. Nothing is prescribed, and everything has to be evidenced.
One route is a compliance check. The other is a design tool that happens to produce compliance.
Glazing and free area in the simplified method
The simplified method works on two numbers per dwelling type.
The first is the maximum area of glazing, expressed against the floor area of the room or dwelling and varying by the orientation the glazing faces. East, south and west facing glazing is constrained more tightly than north facing, because that is where the gain comes from.
The second is the minimum free area of openings available for purging heat. Free area is the clear opening measured through the window, measured through the frame rather than as the glazed area, which is where the first round of errors usually appears. A tilt-and-turn window reporting its full leaf area as free area will not survive a building control check.
Cross-ventilation changes the requirement substantially. A dwelling with openings on opposite facades can meet the free area requirement with less opening area than a single-aspect flat, so the plan form, not the window schedule, carries the decision. Single-aspect south or west facing flats are the hardest case in the simplified method and the most common reason a scheme moves to modelling.
Inputs and outputs of dynamic modelling
A TM59 assessment needs a geometry model, construction build-ups with U-values and g-values, occupancy and internal gain profiles, a ventilation strategy with opening schedules, and the right weather file. Design summer year data for the relevant location is used rather than a typical year, because the point is to test a hot year.
The criteria differ by ventilation type. For naturally ventilated homes, TM59 applies the CIBSE TM52 hours of exceedance criterion to living rooms, kitchens and bedrooms, and adds a separate night-time test for bedrooms based on operative temperature during sleeping hours. For mechanically ventilated homes a fixed operative temperature threshold applies across occupied hours.
Two inputs decide most results. Opening schedules, because a model that assumes windows open all night when the occupant would not do so is optimistic and reviewers know it. And shading, because external shading changes the answer far more than glass specification does. If the model relies on a brise soleil to pass, that brise soleil belongs on the planning drawings and in the condition schedule.
Sample rooms are chosen to represent the worst cases: top floor, single aspect, west or south facing, smallest floor area for the occupancy. Modelling a comfortable middle-floor north-facing flat proves nothing. The reporting of TM59 results has its own conventions, and a model that passes can still be sent back if the room selection is not justified.
Where the simplified method runs out
The simplified method is unavailable in more situations than its name suggests.
- Where the design cannot meet the glazing or free area limits without changing the architecture
- Where openings cannot be relied on at night because of noise, security, pollution or falls from height
- Where the building type falls outside the residential scope of Approved Document O
- Where a planning authority requires a modelled assessment under local policy regardless of the Building Regulations route
That last point catches people. Approved Document O is a Building Regulations matter, decided by building control after planning permission. A planning condition requiring an overheating assessment is a separate obligation, and satisfying one does not discharge the other.
Noise, security and the openings you cannot rely on
An opening only counts if it can actually be opened. Approved Document O treats noise as a constraint on the ventilation strategy: where windows would have to be kept shut at night to keep internal noise at acceptable levels, the free area behind them cannot be claimed.
The same logic applies to security. A ground floor or accessible window with a restrictor has a reduced free area, and the reduced figure is the one to use. Windows above ground level need guarding where the opening allows a fall, and the guarding has to be drawn, not assumed.
Insect screens reduce free area. Air quality is a further constraint on a site inside an air quality management area, because a strategy relying on open windows facing a busy road sits awkwardly with the air quality evidence submitted alongside it. These constraints tend to arrive together on exactly the sites that are hardest to cool, which is the honest reason so many urban flats end up modelled with mechanical cooling as a fallback.
Evidence a planning authority will accept
At planning stage the expectation is set by local policy rather than by the Approved Document. In London, the London Plan cooling hierarchy requires major development to reduce heat risk in sequence before any active cooling is proposed, and a modelled assessment is the normal way of demonstrating it. Outside London, requirements vary by adopted plan, and some authorities ask only for a narrative statement.
What an officer wants to see is consistent. The route taken and why. The rooms assessed and why they are the worst case. The mitigation relied on, with each measure identified on a drawing. A statement of what happens if a measure is lost at technical design stage.
Councils usually expect a suitably qualified author for this work, and professional review is strongly advised. Where the question is who holds the right competence, the position on who can prepare an overheating assessment to TM59 is worth reading before appointing.
Choosing the route at concept stage
Run the test early with three questions.
- Are any dwellings single aspect facing east, south or west, and if so, what proportion of the scheme do they represent?
- Does the design intent depend on glazing areas above the simplified limits for those orientations?
- Is the site in a high risk location, in an air quality management area, or on a road noisy enough to constrain night opening?
Two yes answers point to modelling. Commit to it then, while shading, balcony overhangs and plan depth can still change, rather than at technical design when the only remaining levers are glass specification and mechanical cooling.
The cost comparison usually favours modelling on anything other than a small, generously cross-ventilated scheme, because the simplified method's real price is paid in lost glazing rather than in fees. A drafted overheating risk assessment can set out the strategy, the route chosen and the mitigation schedule ready for a qualified assessor to complete and check.
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 rather than legal or technical advice, so check the current Approved Document and your authority's adopted policy before relying on it.
Frequently asked questions
Does Part O apply to conversions and change of use?
Approved Document O applies to new residential buildings, including dwellings, residential institutions and student accommodation. Material changes of use that create new dwellings can engage the overheating requirement, but the position depends on the work involved, so confirm the scope with building control at an early stage rather than assuming the Approved Document applies in full.
What is free area and how is it measured?
Free area is the clear opening through which air can pass when a window or vent is fully open, measured through the opening rather than as the frame size or the glazed area. Restrictors, insect screens and the opening geometry of tilt-and-turn or top-hung windows all reduce it, and the reduced figure is the one that counts towards the simplified method.
Can a scheme pass Part O but still fail at planning?
Yes. Approved Document O is a Building Regulations matter determined by building control after permission is granted. Planning authorities set their own requirements through adopted policy, and in London the London Plan cooling hierarchy expects major development to demonstrate heat risk reduction in sequence. The two obligations run separately and both have to be met.
Which rooms should a TM59 assessment model?
The worst cases rather than a representative average. That normally means top floor, single aspect dwellings facing south or west, with the smallest floor area for the assumed occupancy, plus any room with unusual glazing or restricted opening. A model that only reports comfortable north-facing middle-floor rooms will be sent back for a wider sample.
Why does single aspect design cause so much trouble?
Cross-ventilated dwellings can purge heat through openings on opposite facades, so the simplified method allows them a smaller opening area. A single aspect flat has to achieve the whole free area requirement on one elevation, which is difficult on a south or west frontage and is the most common reason a scheme moves to dynamic modelling.
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.