Overheating under Part O or a dynamic model
The simplified Part O route is a table you pass or fail. TM59 modelling tells you which room overheats and why. Planning policy usually requires the second.
The simplified method in Approved Document O and dynamic thermal modelling to CIBSE TM59 are two routes to the same building regulations outcome, and they are not interchangeable at planning stage. The simplified method is a set of tabulated limits on glazing area and opening area that you either satisfy or do not. Dynamic modelling simulates the dwelling hour by hour against a weather file and reports how often it overheats.
If your local plan requires an overheating assessment as part of the application, or applies a cooling hierarchy, the simplified method will not answer it. You will be modelling. Knowing that early changes who you appoint and what the glazing schedule looks like.
What the simplified method covers
Approved Document O offers a route to compliance based on limiting unwanted solar gains in summer and providing an adequate means of removing excess heat. In practice it works as two checks against tables: maximum glazing area as a proportion of floor area, and minimum free area of openings for removing heat, with different figures for cross-ventilated and non-cross-ventilated dwellings and for locations categorised as higher or moderate risk of overheating.
Its attraction is that it needs no simulation, no weather file and no specialist. A competent designer with the glazing schedule and the floor plans can work through it.
Its limitation is that it constrains the architecture directly. A flat with glazing on one elevation only, a deep plan, or large south or west glazing will fail the tables, and the remedy is a smaller window rather than a cleverer analysis.
What dynamic modelling covers
Dynamic thermal modelling simulates the building's thermal behaviour through a summer against a defined weather file for the location, with occupancy and internal gain profiles, window opening behaviour, shading and ventilation all specified.
The domestic methodology is CIBSE TM59. For predominantly naturally ventilated homes it applies two criteria: a limit on the hours of exceedance across living rooms, kitchens and bedrooms through the May to September period, and a separate limit on night-time bedroom temperatures above 26 degrees. For mechanically ventilated homes a single criterion on hours above 26 degrees in occupied rooms applies instead.
The advantage is that it tells you something real. It shows which rooms fail, by how much, and which intervention fixes them, and it can credit shading, thermal mass and ventilation strategies that the tables cannot see.
| Simplified method | Dynamic modelling to TM59 | |
|---|---|---|
| Basis | Tabulated glazing and opening area limits | Hourly simulation against a weather file |
| Specialist needed | No | Yes |
| Credits shading, mass and ventilation strategy | Very little | Yes |
| Room by room result | No | Yes |
| Answers a planning policy requirement | Rarely | Usually |
| Available where windows cannot be opened at night | No | Yes |
| Cost and programme | Low | Higher, and needs the design fixed enough to model |
Where the simplified route is unavailable
This is the part worth checking before the design develops. The simplified method cannot be used where the conditions it assumes do not hold, and the common cases are:
- noise means windows cannot be opened at night at the relevant facade
- air quality at the facade makes night ventilation undesirable
- security or the risk of falls restricts the openable area, particularly at low level or on balconies
- the dwelling form falls outside the arrangements the tables cover
- the scheme uses a ventilation strategy the tables do not contemplate
The noise case is the one that recurs. A flat on an urban main road frequently needs closed windows at night for acoustic reasons and open windows for thermal reasons, and the two assessments have to be reconciled rather than written separately by two consultants who never speak. Where a noise assessment concludes that windows must remain shut, the overheating assessment has to be modelled with them shut.
Planning policy requirements beyond the regulations
Part O is a building regulations matter. Planning policy is a separate question, and in several authorities, London boroughs in particular, adopted policy requires an overheating assessment at application stage and requires the applicant to work through a cooling hierarchy: reduce the heat entering the building, then manage the heat within it, then provide passive ventilation, then mechanical ventilation, and only then active cooling.
A hierarchy policy cannot be answered with a glazing area table, because the policy asks you to demonstrate the sequence of measures considered and rejected. So on those schemes the modelling is doing planning work as well as regulatory work, and the report needs a section that walks the hierarchy explicitly.
Check the local plan and the local validation list before deciding the route. Committing to the simplified method and then discovering a policy requirement is a late and expensive reversal, because the glazing may already be fixed in the elevations you have consulted on.
Competence for the modelling
Dynamic modelling is only as good as the modeller. The competence questions to ask are specific: which software, what experience of TM59 specifically rather than of energy modelling generally, and whether the modeller will release the model file.
The methodology has a great deal of judgement in it, and the note on who can prepare an overheating assessment to TM59 sets out what to look for in more detail. Broadly: a building services background, familiarity with the weather files and their selection, and enough exposure to real schemes to know which inputs matter.
The inputs that change the result
Six, in rough order of influence.
- The weather file, and which year and location it is for. Using a file for a cooler location than the site is the single largest source of optimistic results.
- Window opening assumptions, including whether windows can be opened at night and by how much.
- Shading, both external fixed shading and internal blinds, and whether blind use is assumed or modelled.
- Internal gains and occupancy profiles, particularly for small single aspect flats.
- Corridor and riser temperatures in buildings with communal heating, because uninsulated distribution pipework raises corridor temperatures and with them the flats.
- Glazing specification, especially the solar factor of the units.
State every one of these in the report. A TM59 result presented without the weather file named is not an assessment, it is an assertion, and a reviewing officer with any experience will say so.
Presenting the outcome to the council
Lead with the method, the location, the weather file and the criteria applied, then give the results room by room in a table with pass or fail against each criterion, then the mitigation adopted, then the residual position for any room that still fails and why that is acceptable.
Where the policy applies a cooling hierarchy, structure the mitigation section as the hierarchy itself so the officer can tick it off. And keep the assumptions consistent with the energy and acoustic submissions, because contradictions between the three documents are the fastest route to a holding objection. The note on reporting TM59 results covers the presentation in more detail.
An Overheating Risk Assessment is a report councils expect a suitably qualified author to prepare, and professional review is strongly advised. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Browse the 66 PlanScope report types to see which documents the assessment should sit alongside. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What is the simplified method in Approved Document O?
A route to compliance based on tabulated limits: maximum glazing area as a proportion of floor area to limit solar gains, and minimum free opening area to remove excess heat, with different figures for cross-ventilated and non-cross-ventilated dwellings and for higher and moderate overheating risk locations. No simulation or specialist is needed.
When can the simplified method not be used?
Where its assumptions do not hold. The common cases are noise or air quality preventing night ventilation at the relevant facade, security or fall restrictions limiting openable area, a dwelling form outside the arrangements the tables cover, and ventilation strategies the tables do not contemplate. An urban main road flat frequently falls into the first of those.
What criteria does TM59 apply?
For predominantly naturally ventilated homes, a limit on hours of exceedance across living rooms, kitchens and bedrooms through the May to September period, plus a separate limit on night-time bedroom temperatures above 26 degrees. For mechanically ventilated homes, a single criterion on hours above 26 degrees in occupied rooms applies instead.
Does planning policy require dynamic modelling?
Often, and increasingly. Where adopted policy requires an overheating assessment at application stage or applies a cooling hierarchy, a glazing area table cannot answer it, because the policy asks you to demonstrate the sequence of measures considered. Check the local plan and the validation list before deciding the route, since glazing gets fixed early.
Which modelling input matters most?
The weather file and the location it is drawn from. Using a file for a cooler location than the site is the largest single source of optimistic results. After that: window opening assumptions, shading and blind use, internal gains and occupancy, corridor temperatures where there is communal heating, and the solar factor of the glazing.
How should the results be presented?
Method, location, weather file and criteria first, then results room by room in a table with pass or fail against each criterion, then the mitigation adopted, then the residual position for any failing room. Where a cooling hierarchy applies, structure the mitigation section as the hierarchy so the officer can work through it.
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.