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Part O Overheating Mitigation Requirements
Dynamic overheating criteria, glazing limits by orientation, cross-ventilation requirements, and shading factors for new residential buildings, referencing Approved Document O: Overheating (2021 edition, effective 15 June 2022).
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 Purge Ventilation Calculator for openable window free areas.
Always check the latest edition of Approved Documents on the Planning Portal (planningportal.co.uk) as regulations are updated periodically.
Next up: related pages
Frequently Asked Questions
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.
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.
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.
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.
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.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.