When is it required?
Part O of the Building Regulations applies to new residential buildings within its scope. Government guidance confirms that it does not apply to extensions or material changes of use. That does not mean overheating can be ignored in a planning application: London housing-design policy and local validation requirements can require consideration of heat risk more widely. [1] [2] [3]
Check the purpose of the assessment before commissioning it. A report intended to support planning should address the relevant planning brief; a building-regulations submission must follow the applicable compliance route and assumptions.
How is the design assessed?
Approved Document O provides a simplified method and a dynamic thermal-modelling method. The appropriate route depends on the building and circumstances. The assessment must consider limiting unwanted solar gains and removing excess heat, alongside whether the proposed measures can actually be used. [4] [1]
Ask the assessor to identify the rooms tested, the drawing revisions, glazing and shading assumptions, ventilation arrangements and the weather data used. These details make it possible to understand why the design performs as reported and what must be preserved during later changes.
Windows cannot be treated as an abstract opening
Government guidance addresses noise, safety, security and other practical restrictions affecting the use of windows and ventilation openings. A model that assumes unrestricted opening needs to be reconciled with the real location and operation of the building. [1]
Coordinate the assessment with the acoustic report and the architectural details. Ask for a clear explanation where one study assumes closed windows while another relies on opening them. The final design needs a workable combined solution, not separate reports with incompatible assumptions.
Which version of TM59 should be used?
CIBSE published a revised TM59 methodology in July 2026. However, its publication page expressly retains TM59:2017 for the Part O thermal-modelling compliance route, while recommending TM59:2026 as industry best practice. Publication of the new methodology did not automatically amend Approved Document O. [5]
The appointment should therefore identify the applicable regulatory method and any additional planning or design-quality assessment. CIBSE says it is discussing incorporation of the updated methodology and weather files with the Building Safety Regulator; no implemented replacement date is stated on that page. [5]
Who prepares it, and when?
Use an assessor with relevant building-physics or thermal-modelling experience, working with the architect and services designers. Start while glazing, shading and ventilation can still be changed. Westminster's energy checklist illustrates the need to coordinate the overheating assessment with the wider energy strategy. [3]
Before ordering windows or cooling equipment, ask the assessor to review material substitutions and design changes. Retain the final report and the specifications that support its conclusions so the construction team can deliver the assessed arrangement.