When does Part O apply?

Part O (Approved Document O: Overheating, 2021 edition) applies to all new residential buildings in England from 15 June 2022. It covers new-build houses, flats, and residential buildings (including student accommodation and care homes). It does not apply to existing dwellings, extensions, or non-residential buildings. Its purpose is to reduce the risk of high indoor temperatures during summer without reliance on energy-intensive mechanical cooling.

What is the simplified method?

The simplified method (AD-O Section 3) sets maximum glazing area limits as a percentage of floor area, depending on orientation, location, cross-ventilation availability, and shading. If the glazing in each room does not exceed the limit in the tables, no further assessment is needed. This method is conservative — most well-designed dwellings can use it without needing the dynamic method.

What is the dynamic method?

The dynamic thermal modelling method (AD-O Section 4) uses TM59 (CIBSE) criteria to demonstrate compliance. Bedrooms must not exceed 26°C for more than 1% of annual occupied hours (and hours where the operative temperature exceeds 26°C should be no more than 32.7 hours). Living rooms must not exceed 28°C for more than 1% of annual occupied hours. This method allows more glazing if adequate mitigation measures are demonstrated.

Does Part O require openable windows?

Part O requires that dwellings have adequate means to remove excess heat. In most cases this means openable windows providing cross-ventilation. Where noise or security prevents windows being opened (e.g. ground floor in urban areas, near busy roads), alternative strategies such as mechanical ventilation or shading must be provided. The free area of openable windows for purge ventilation must be sufficient per the tables.

Does Part O apply in Scotland, Wales, or Northern Ireland?

Part O applies only in England. Wales introduced similar requirements through its own AD-O. Scotland addresses overheating through Technical Handbook Section 6 (Energy) with guidance on solar gain and ventilation. Northern Ireland does not yet have an equivalent to Part O, though overheating is addressed through energy efficiency requirements. The requirements differ between jurisdictions.

Part O Overheating Mitigation Requirements

Simplified Method — Maximum Glazing as % of Floor Area (AD-O Table 3.1)

These are indicative maximum glazing-to-floor-area ratios for rooms that achieve cross-ventilation. Values are lower for single-aspect rooms and high-risk locations.

Glazing Orientation	Cross-Ventilated Room	Single-Aspect Room	Notes
North (315°-45°)	~25-30%	~20-25%	Lowest solar gain
East (45°-135°)	~20-25%	~15-20%	Morning sun exposure
South (135°-225°)	~15-22%	~12-18%	High midday/afternoon gain
West (225°-315°)	~15-20%	~10-15%	Highest risk — late afternoon sun when outdoor temp peaks

Exact percentages depend on location zone, shading, and g-value of glazing. Refer to AD-O Tables 3.1-3.4 for specific values.

Dynamic Method — TM59 Overheating Criteria (CIBSE TM59:2017)

Criterion	Threshold	Applies To
Criterion A: Hours above threshold	≤ 3% of occupied hours above adaptive comfort limit	All habitable rooms (naturally ventilated)
Criterion B: Bedroom night-time	≤ 32.7 hours above 26°C (operative temp, 10pm-7am)	Bedrooms only
Fixed threshold (mechanically cooled)	≤ 1% of hours above 26°C (bedrooms) or 28°C (other)	Where mechanical cooling provided

Cross-Ventilation Requirements

Requirement	Detail
Cross-ventilation definition	Openable windows on opposite or adjacent walls allowing through-air movement
Single-aspect room	Openings on one wall only — higher overheating risk, lower glazing limits
Free area of openable windows	Typically ≥ 1/20th floor area for purge ventilation
Security considerations	Where windows cannot be opened safely, mechanical alternatives required

Shading Factors

Shading Type	Approx. Solar Gain Reduction
External shutters / brise-soleil	60-80%
External awning / overhang	40-60%
Mid-pane blinds (integrated)	30-50%
Internal blinds / curtains	10-30%
Low g-value glazing (solar control)	20-40% (g-value 0.3-0.5 vs standard 0.6-0.7)

Related Calculators

Use the U-Value Calculator for thermal performance, or the Ventilation Calculator for ventilation design.

Always check the latest edition of Approved Documents on the Planning Portal (planningportal.co.uk) as regulations are updated periodically.

Part O Overheating Mitigation Requirements

Simplified Method — Maximum Glazing as % of Floor Area (AD-O Table 3.1)

Dynamic Method — TM59 Overheating Criteria (CIBSE TM59:2017)

Cross-Ventilation Requirements

Shading Factors

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How We Calculate This

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