Part O Overheating Calculations

Part O of the Building Regulations (Overheating) came into effect in June 2022. Its purpose is to mitigate the increasing risk of overheating in new residential dwellings and certain other building types in the UK. This addresses concerns stemming from climate change, increased glazing, and improved airtightness in modern construction, all of which can contribute to internal heat build-up.

Part O requires a design assessment to ensure that new residential buildings (and some other specified buildings) are designed to limit unwanted solar gain and provide adequate means for the safe removal of excess heat. This typically involves evaluating individual rooms to ensure they meet the criteria, preventing uncomfortable and potentially dangerous internal temperatures.

Overheating assessments under Part O are a mandatory requirement for all new residential dwellings, including houses, flats, and student accommodation. They’re also important for demonstrating compliance during significant renovations or changes of use in existing properties where alterations might impact overheating risk. These calculations are best carried out at the design stage, so any issues can be resolved before construction begins.

Part O offers two routes to compliance: the Simplified Method (“Route A”) and Dynamic Thermal Modelling (“Route B” / CIBSE TM59).

This method provides an initial, straightforward assessment of overheating risk based on prescriptive rules and tables. It’s often suitable for simpler, less complex projects, and helps identify potential Part O compliance issues early in the design process.

For more complex projects, those requiring in-depth analysis, or projects that have failed the simplified method, a CIBSE TM59 assessment is recommended. This advanced method uses sophisticated software simulations to build a detailed thermal model of the dwelling, providing a comprehensive evaluation of overheating risk under realistic conditions.

CIBSE has recently (July 2026) published TM59:2026, a major revision to the dynamic thermal modelling methodology, introducing future (2050s) climate data, revised bedroom and communal space criteria, and a new staged, passive-first assessment framework. Approved Document O currently still references TM59:2017, so this remains the standard route for most projects today – but TM59:2026 is expected to be formally adopted in due course, and some local authorities may already accept it voluntarily.