Required for All New Dwellings Since June 2022
Part O Overheating Assessments
Part O compliance for new homes. Most pass using the simplified method. Dynamic simulation available for complex designs.
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Part O Overheating Assessments
What is Part O?
Building Regulation requiring new homes to limit overheating risk without mechanical cooling.
Do I need Part O?
Yes - all new dwellings in England since 15 June 2022. Houses, flats, conversions.
Simplified or dynamic?
Try simplified first - most homes pass. Dynamic if simplified fails or design is complex.
How much does it cost?
Simplified from £95. Dynamic modelling from £250.
What makes designs fail?
Large south/west glazing, single-aspect layouts, limited ventilation, noise constraints.
Can it be done with SAP?
Yes - we normally complete SAP and Part O together using the same drawings.
Need Part O alongside your SAP? We complete both together for efficiency.
What Is Part O?
Part O is the Building Regulation covering overheating in new residential buildings. It ensures homes remain comfortable during hot weather without relying on energy-intensive air conditioning.
Introduced on 15 June 2022, Part O recognises that modern well-insulated homes can overheat in summer, particularly those with large south or west-facing glazing. The regulation requires designers to limit overheating risk through passive measures - glazing limits, solar shading, cross-ventilation, and thermal mass.
Part O applies to new dwellings, conversions creating new dwellings, and buildings with a material change of use to residential. It must be demonstrated alongside SAP as part of Building Control submission.
Who Needs a Part O Assessment?
Part O applies to:
- All new-build houses
- All new-build flats and apartments
- Conversions creating new dwellings (barn conversions, commercial to residential)
- Buildings with material change of use to residential
- Extensions that create a new dwelling
- Replacement dwellings (demolish and rebuild)
Part O does not apply to existing homes being extended (unless creating a new dwelling) or to non-residential buildings (these may need TM52 instead).
Simplified Method or Dynamic Simulation?
Part O offers two compliance routes. We recommend starting with simplified - most homes pass.
What Makes Designs Fail Part O?
These design features commonly trigger Part O issues:
Glazing Issues
- ✓ Large south or west facing windows
- ✓ Roof lights and lanterns
- ✓ Floor-to-ceiling glazing
- ✓ Glazing exceeding Part O limits
Ventilation Issues
- ✓ Single-aspect flats (one external wall)
- ✓ Limited openable window area
- ✓ Security concerns preventing window opening
- ✓ Noise constraints on ventilation
Layout Issues
- ✓ Bedrooms on upper floors with large glazing
- ✓ Open-plan living areas with extensive glazing
- ✓ Limited thermal mass (lightweight construction)
- ✓ Deep plan buildings with poor cross-ventilation
If any of these apply, dynamic simulation may be needed. We'll advise after reviewing your plans.
Part O Assessment Pricing
Start with simplified - it's quicker and cheaper. Move to dynamic only if needed.
Simplified Method
From £95
Prescriptive compliance check. Same-day results for most projects.
TM59 Dynamic
From £250
Full dynamic simulation when simplified doesn't work.
SAP + Part O Bundle
From £220
Combined package for new dwellings needing both assessments.
How Part O Assessment Works
We aim to demonstrate compliance using the simplest method possible.
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1
Send us your drawings
Floor plans, elevations, glazing schedule, and ventilation strategy. If you're also getting SAP, we use the same documents.
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2
We check simplified first
The simplified method is quickest. If your design passes, you're done. If not, we'll explain why and recommend dynamic if appropriate.
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3
Dynamic modelling if needed
If simplified fails or your design is complex, we build a thermal model and simulate performance through a design summer year.
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4
Receive your report
Clear pass/fail result with Building Control documentation. Mitigation recommendations if any rooms fail.
Common Part O Mitigation Strategies
If your initial assessment fails, these changes often bring designs into compliance:
- Reduce glazing - Often the most effective fix. Even small reductions in south/west glazing help.
- Add solar shading - External shutters, brise soleil, or deep reveals. More effective than blinds.
- Improve ventilation - Larger openable areas, add windows for cross-ventilation.
- Change glazing specification - Solar control glass reduces solar gains while maintaining views.
- Increase thermal mass - Exposed concrete, masonry walls, solid floors absorb heat.
- Relocate sensitive rooms - Move bedrooms away from large south/west glazing.
We recommend the most cost-effective solutions for your specific design.
Part O FAQ
Can I do Part O at the same time as SAP?
Does Part O apply in Scotland/Wales/Northern Ireland?
What if my client wants air conditioning?
Do extensions need Part O?
What's the difference between Part O and TM59?
Ready to get your Part O assessment?
All Part O assessments accepted by Building Control nationwide.