Part O Overheating: Complete Compliance Guide
Everything you need to know about Part O of the Building Regulations - when it applies, how to comply, and strategies to reduce overheating risk.
What is Part O?
Part O is the section of the Building Regulations (England) that addresses overheating risk in new residential buildings. Introduced on 15 June 2022, it requires new dwellings to be designed to:
- Limit unwanted solar gains in summer
- Provide adequate means to remove excess heat
- Protect occupants from the health risks of overheating
Why Was Part O Introduced?
Climate change is increasing summer temperatures. Highly insulated, airtight modern homes retain heat. Overheating is a serious health risk, particularly for vulnerable occupants. Part O ensures new homes are designed to stay cool.
When Does Part O Apply?
Part O applies to:
- All new dwellings (houses, flats, bungalows)
- Conversions creating new dwellings
- Residential buildings (care homes, student accommodation)
It does NOT apply to existing homes (unless converted), extensions to existing homes, or non-residential buildings.
Compliance Methods
Part O offers two compliance routes:
1. Simplified Method
Prescriptive approach using:
- • Maximum glazing limits
- • Shading requirements
- • Minimum ventilation openings
- • Cross-ventilation rules
Best for: Standard designs
2. Dynamic Modelling
Detailed simulation showing:
- • Hour-by-hour temperatures
- • Overheating hours below threshold
- • CIBSE TM59 criteria
- • Specific room performance
Best for: Complex designs
Overheating Risk Factors
Higher Risk
- London / South East location
- South/West facing bedrooms
- Single aspect flats
- Large glazing areas
- Top floor units
- No external shading
Mitigation Strategies
- External shading (overhangs)
- Reduced glazing ratios
- Low g-value glazing
- Cross-ventilation
- Purge ventilation
- Thermal mass
Part O vs TM59
Part O and CIBSE TM59 both address overheating, but they're different:
- Part O is a Building Regulations requirement (mandatory for all new dwellings in England)
- TM59 is a CIBSE methodology often required by planning authorities, especially in London
Some projects need both assessments. We can advise which applies to your project.