Part O - Overheating

Part O – Overheating sets out requirements to limit overheating in new residential buildings.
Introduced in 2022, it ensures that homes are designed to remain safe and comfortable in hot weather by minimising internal temperature rise.

Part O applies to all new dwellings, including flats and houses, and requires every developer to demonstrate that passive measures such as natural ventilation and shading have been considered before resorting to mechanical cooling.

The earlier the better.
Overheating should be assessed at the design stage, ideally alongside SAP 10.2 and Part L 2021 assessments.

Early modelling allows window sizing, orientation, and g-values to be optimised before elevations are fixed. Late assessments often lead to costly redesigns or restricted glazing areas.

No, not currently
Part O applies only to new residential buildings. It does not apply to:

• Extensions or conservatories added after completion
• Conversions or change-of-use projects

However, many local authorities now expect overheating risks to be considered voluntarily for larger refurbishments.

There are two recognised routes to demonstrate Part O compliance:

Simplified Method

A prescriptive checklist with fixed limits for:

• Glazing-to-floor-area ratios
• Window orientation
• Opening sizes and ventilation pathways

Best for simple dwellings with modest glazing and good cross-ventilation.

Dynamic Thermal Modelling (CIBSE TM59)

A detailed simulation using specialist software (e.g. IES VE, DesignBuilder) that tests real weather data, occupancy patterns, and shading.

Required for complex, urban, or heavily glazed designs, or where noise, pollution, or security limit natural ventilation.

Start by reviewing the most glazed room in each dwelling.
If the glazing-to-floor ratio exceeds Simplified Method limits or if there’s limited cross-ventilation you’ll need the Dynamic TM59 approach.

To complete either the Simplified or Dynamic TM59 route, we typically require:

• Site and floor plans with orientation
• Elevations and sections
• Window and door schedules (sizes & openings)
• Fabric U-values
• Ventilation strategy (natural or mechanical)
• Details of external shading (louvres, overhangs, blinds, balconies)
• Noise or pollution data (if applicable)

Supplying this information early helps ensure accurate, compliant results.

Key variables include:

• Orientation and exposure to the sun
• Glazing area and g-value (solar factor)
• Cross-ventilation opportunities
• External shading from balconies or overhangs
• Thermal mass and room depth
• Urban noise and pollution constraints (limiting window use)

Designers should integrate these factors holistically with Part L 2021 energy strategy to balance heating and cooling needs.

Not directly.
Mechanical Ventilation with Heat Recovery (MVHR) improves air quality and reduces heat loss in winter but does not significantly reduce summer overheating.

Boost functions can increase air change rates but cannot offset high internal gains when outdoor air is warm.
If an MVHR system actively cools incoming air, it’s classed as mechanical cooling, which can only be used once all passive measures have been exhausted.

The g-value (solar factor) of glazing determines how much solar radiation enters the building.

• Low g-values (≤ 0.4) reduce solar gain and help with overheating.
• Higher g-values (0.5–0.6) improve winter heating efficiency but increase risk in summer.

Balancing g-values with orientation and shading design is critical to achieving both Part L and Part O compliance.
Our assessors will review glazing specification to ensure optimal year-round comfort.

Part O requires that overheating mitigation does not compromise:

• Night-time noise (especially for bedrooms)
• Pollution levels (near roads or industrial areas)
• Security and safety (particularly at ground level)

In such cases, TM59 Dynamic Modelling allows for alternative solutions—such as restricted openings, secure louvres, or acoustic vents—while still demonstrating compliance.

Overheating assessments are reviewed by Building Control or an Approved Inspector.
You’ll typically need to demonstrate compliance at:

• Design Stage to secure approval before construction
• As-Built Stage confirming that installed openings, glazing, and shading match the approved design

Energy Report Limited provides both stages with all necessary documentation.

We offer full Part O Overheating Compliance services, including:

• Simplified Method and Dynamic CIBSE TM59 Modelling
• Integration with SAP 10.2 and Part L 2021 assessments
• Early-stage design advice to minimise risk and retrofit costs
• Collaboration with architects to optimise orientation, shading, and ventilation strategies
• Clear, evidence-based reports ready for Building Control submission

Our proactive, design-led approach ensures that your homes meet thermal comfort standards while remaining efficient, buildable, and compliant.

Fees for an Overheating assessment on a single dwelling start from £220 plus VAT for a Simplified Analysis and starting at £440 plus VAT for a Dynamic Assessment