Which buildings
Approved Document O applies to new residential buildings only: dwellings (houses and flats); institutional residential buildings such as care homes; and other residential buildings such as student accommodation. Shared communal rooms and common spaces in buildings with more than one home are included.
Route 1: the simplified method
The simplified method (Section 1) is suitable for any building within the scope of Part O. It works in two steps: limit solar gains in summer, and provide a way to remove excess heat.
- Location: each building is either in a "high risk" location (urban and some suburban parts of London, listed by postcode in Appendix C) or a "moderate risk" location (the rest of England). Appendix C also notes that some central Manchester postcodes may have elevated night-time temperatures.
- Cross-ventilation: whether each home has openings on opposite façades. Openings on façades that aren't opposite, as in a corner flat, don't count.
- Limiting solar gains: maximum glazing areas, as a percentage of floor area, for the building and for the most glazed room, depending on the orientation of the most glazed façade, the location and cross-ventilation (Tables 1.1 and 1.2). Buildings in the high risk location also need shading.
- Removing excess heat: minimum free areas of openings such as windows, so heat can be removed by ventilation.
Route 2: dynamic thermal modelling (TM59)
Section 2 allows dynamic thermal modelling following CIBSE TM59, Design methodology for the assessment of overheating risk in homes. Building control is given a report showing the building passes TM59's assessment. Approved Document O adds limits on the modeller's choices, for example:
- when a room is occupied during the day (8am to 11pm), windows start to open above 22°C, are fully open above 26°C, and close again as the temperature falls;
- at night (11pm to 8am), openings are modelled as fully open only on the first floor or above, where not easily accessible, and when the room is above 23°C at 11pm;
- ground floor or easily accessible rooms are modelled with windows closed at night;
- internal blinds and curtains aren't counted as reducing solar gains.
Approved Document O says dynamic modelling may give more design flexibility for homes with very high insulation and airtightness, sites not well represented by the two simplified locations, and homes heavily shaded by neighbouring buildings or landscape.
When the simplified method may not work
Section 3 requires the overheating strategy to be usable in practice: allowing for noise, pollution, security, and protection from falling and entrapment. If, for example, external noise means windows are likely to be closed at night, the simplified method's free areas can't be relied on and dynamic modelling should be used. Approved Document O treats bedroom noise above 40 dB LAeq,T averaged over 11pm to 7am, or above 55 dB LAFmax more than 10 times a night, as making that likely.
At a glance
| Simplified method | Dynamic modelling (TM59) | |
|---|---|---|
| What it checks | Glazing limits, shading in high risk locations, minimum free areas | Predicted indoor temperatures against TM59's criteria |
| Inputs | Location, orientation, glazing and floor areas, cross-ventilation | A model of the building, weather file, occupancy and opening profiles |
| Flexibility | Fixed limits | More design freedom, within Approved Document O's limits |
| Evidence | Compliance checklist (Appendix B) | TM59 report and compliance checklist |
Information for the owner
Regulation 40B requires information about the overheating strategy to be given to the owner, so the home can be used as designed.
This guide summarises Approved Document O; it isn't compliance advice. Your designer or energy assessor will advise which route suits a scheme, and your building control body decides whether it complies.
Sources
- Approved Document O: overheating, 2021 edition (GOV.UK)
- PBC Today: Part O, understanding the new regulations on overheating
Last checked: October 2026.