Approved Document O (overheating) applies to new residential buildings in England: houses, flats, care homes, student accommodation and similar. It came into force on 15 June 2022. You can show compliance in two ways: the simplified method or dynamic thermal modelling to CIBSE TM59. Picking the right one early can save redesign later.
Route 1: the simplified method
This is a check rather than a computer model. It limits the amount of glazing for each home and sets minimum free areas for opening windows, depending on:
- location: parts of London and Manchester are “high risk”, the rest of England is “moderate risk”;
- cross-ventilation: whether each home has openings on more than one side;
- orientation: how much glazing faces the hottest direction.
It suits straightforward houses and flats with sensible glazing. It’s quick and cheap, but it’s strict: large glazed areas or single-aspect flats often fail it.
Route 2: dynamic thermal modelling (TM59)
A 3D model of the building is run through a year of hourly weather data to predict room temperatures, using the CIBSE TM59 method. It has to show that rooms stay within set limits, including bedrooms at night.
TM59 gives far more design freedom. You can keep bigger windows if you add shading, better glazing or more ventilation. It’s the usual route for apartment blocks, single-aspect flats, heavily glazed designs and care homes, and whenever the simplified method fails.
Send your drawings and we’ll tell you whether the simplified method will pass or TM59 is the better option, then quote for it.
Things Part O also checks
- Noise: if windows have to stay shut at night because of road or rail noise, you can’t rely on them for cooling.
- Security and safety: openings used for cooling must be secure and safe from falls.
- Pollution: openings near busy roads may not count.
- Information for occupants: the owner must be told how the overheating strategy works, for example which windows to open.
When to do it
As early as possible, ideally before planning or at least before the window schedule is fixed. Overheating, SAP and ventilation all depend on the same windows, so it’s cheaper to do them together.
Related services: Overheating Assessment, TM59 Assessment, SAP Calculations.
Frequently asked questions
Does Part O apply to conversions?
Part O applies to new residential buildings, including those formed by a material change of use, such as an office converted to flats. Check with your Building Control body for your specific project.
Can I use the simplified method for flats?
Yes, if the flats meet its glazing and ventilation limits. Single-aspect or heavily glazed flats often fail it and need TM59 modelling instead.
What is TM59?
TM59 is a CIBSE method for assessing overheating risk in homes using dynamic thermal modelling. It’s the recognised modelling route for Part O compliance.
Who checks Part O?
Your Building Control body. They’ll want the simplified method calculation or the TM59 report, usually before or during construction.
Last reviewed: October 2026. General guidance for England only. Rules change and every site is different, so get project-specific advice before relying on it.
More guides in the Developer Knowledge Centre · Services: Overheating Assessment, TM59 Assessment