Completions & Planning · Mandatory · Pre-Construction

Overheating Assessment Record (TM59 / Approved Document O)

The overheating assessment record required for all new residential buildings in England under Approved Document O (in force June 2022). Documents compliance via the simplified method or dynamic thermal modelling to CIBSE TM59 criteria, including living area and bedroom temperature thresholds, mitigation measures, and solar shading calculations. Must be completed at design stage and recorded before the completion certificate.

Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. Building Regulations 2010 — Approved Document O remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.

Legal basis	Building Regulations 2010 — Approved Document O (in force 15 June 2022)
Applies to	All new residential buildings in England
Methods	Two methods: Simplified (opening areas and shading provisions) or Dynamic thermal modelling (CIBSE TM59)
Living areas	Less than 1% of occupied hours above 26°C
Bedrooms	≤32 nights above 26°C between 10pm and 7am
Key principle	Assessment must be carried out at design stage — not retrospectively at completion

1. Overheating Assessment — A Mandatory New Requirement for All New Homes

The combination of climate change and increasingly well-insulated housing has created a serious overheating risk in new residential buildings. The UK recorded temperatures above 40°C for the first time during the 2022 heatwave, and climate projections indicate that such events will become more frequent. Homes designed to retain heat efficiently in winter can become dangerously hot in summer — particularly upper-floor flats with large glazed areas and limited cross-ventilation.

Approved Document O was introduced in June 2022 to address this risk. It requires an overheating assessment for all new residential buildings at design stage, with compliance demonstrated before construction begins. The assessment must be carried out at design stage because the mitigation measures — window sizing, orientation, shading, ventilation strategy — are fundamental design decisions that cannot be changed retrospectively once the building is constructed.

Design stage requirement

The overheating assessment must be completed at design stage and submitted with the building control application. Retrospective assessments at completion are not acceptable — by that point, the design decisions that determine overheating performance have already been built and cannot be changed without significant cost and delay.

2. Overheating Assessment Record

The following table sets out the information that the overheating assessment record must contain, covering both the simplified method and the dynamic TM59 modelling method.

Field	Detail
Project reference	Site address, building control reference, plot numbers or unit identifiers, assessor name and qualifications.
Assessor	Name, qualifications, and organisation. For TM59 dynamic modelling, the assessor must be competent in the use of approved dynamic thermal simulation software.
Assessment date	Date of assessment. Must be at design stage — before construction commences.
AD O method used	Simplified method or Dynamic TM59 method. The simplified method can only be used where the building meets specific criteria (location, noise environment, security constraints).
Simplified method	For each habitable room: glazing area as percentage of floor area, opening area as percentage of floor area, cross-ventilation provision, shading provision. Compliance checked against AD O tables. If any room fails, either redesign or use TM59 method.
Dynamic TM59 method	Thermal simulation using approved software (e.g. IES VE, TAS). Weather file: DSY1 2020s 50th percentile for the site location. Results for each habitable room: living areas — percentage of occupied hours above 26°C (must be <1%). Bedrooms — number of nights above 26°C 10pm–7am (≤32 nights).
Mitigation measures	Design measures incorporated to achieve compliance: window sizing and orientation, external shading devices, cross-ventilation strategy, thermal mass, reduced internal gains. Mechanical cooling is a last resort — AD O prioritises passive measures.
Overall compliance	Pass / fail for each habitable room and overall building. If any room fails, the assessment does not comply and design changes are required before building control approval.

3. Common Mistakes

Not carrying out the assessment until completion

The overheating assessment must be completed at design stage. Leaving it until completion is too late — the fundamental design decisions that determine overheating performance (window sizes, orientation, shading, ventilation openings) have already been built. Retrospective mitigation is expensive, disruptive, and may not achieve compliance. Building control bodies should not accept a retrospective assessment for a building that was required to comply with AD O at design stage.

Using the simplified method when it is not applicable

The AD O simplified method can only be used where specific criteria are met — including noise environment and security constraints that do not prevent windows from being opened. In urban locations with high external noise levels, or ground-floor units where security concerns restrict opening windows, the simplified method cannot demonstrate compliance. In these cases, dynamic TM59 modelling is required, which adds time and cost — this must be anticipated in the design programme.

4. Frequently Asked Questions

Does Approved Document O apply to commercial buildings?▾

No — Approved Document O applies to new residential buildings only. For non-domestic buildings, overheating risk is assessed using CIBSE TM52 (The Limits of Thermal Comfort: Avoiding Overheating in European Buildings). Some planning conditions or BREEAM requirements may also require an overheating assessment for mixed-use developments, but the AD O criteria and pass/fail thresholds apply only to residential units.

Can mechanical cooling (air conditioning) be used to comply with AD O?▾

AD O prioritises passive design measures — window sizing, orientation, shading, cross-ventilation, and thermal mass. Mechanical cooling is permitted as a last resort where passive measures alone cannot achieve compliance, but the designer must demonstrate that passive measures have been exhausted first. Over-reliance on mechanical cooling increases energy consumption, running costs for occupants, and carbon emissions — which conflicts with Part L (energy efficiency) objectives.

What weather data should be used for TM59 modelling?▾

The assessment must use the Design Summer Year 1 (DSY1) weather file for the 2020s at the 50th percentile, specific to the site location. This represents a moderately warm summer based on projected future climate data. Using historical weather data or the wrong location file will produce inaccurate results and may not be accepted by building control.

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