What if the glazing that brings natural light into a home also increases overheating risk and puts building control approval in doubt? Generous windows can increase solar gain, but reducing glazing is not the only way to meet the requirements. Part O applies to new residential buildings in England, and part o overheating calculations help show whether a design can limit excess heat and provide suitable evidence for building control.
If you’re deciding between the Simplified Method and dynamic thermal modelling, the right route depends on the building’s location, orientation, ventilation and glazing. This guide explains how the methods work, what the current rules mean in 2026 and what to consider early, before design changes become disruptive. The 2021 edition of Approved Document O remains in force in England. A technical review has been confirmed, but the current requirements remain the basis for planning.
Key Takeaways
- Check whether Approved Document O applies to your project, then identify the evidence building control will need.
- Choose an assessment route that suits the design, from the prescriptive Simplified Method to dynamic thermal modelling.
- Assess glazing, orientation and ventilation together, since changing one can affect the overheating strategy.
- Consider shading and other passive measures early to balance thermal comfort with the intended design.
Understanding Part O: Why Part O Overheating Calculations are Essential
Approved Document O sets out how new homes in England should limit unwanted solar gain and remove excess heat through adequate ventilation. It forms part of the wider framework of Building regulations in the United Kingdom. Its purpose is to help protect occupants from harmful indoor temperatures as summer heat and heatwave risks increase.
The current 2021 edition took effect in England on 15 June 2022. SAP calculations assess a dwelling’s energy performance, but they do not demonstrate Part O compliance on their own. The overheating assessment is separate. Coordinating SAP calculations and part o overheating calculations early can help developers assess energy performance and thermal comfort together, and spot potential conflicts before the design is fixed. This matters particularly when glazing choices affect both daylight and the amount of heat entering the home.
Location also matters. The Simplified Method distinguishes between moderate and high-risk areas, and some locations in London are classed as high risk. This can affect the glazing limits and the assessment route available to a project. For developments in Bedford, Cambridge or London, check the site’s risk category early, before confirming window layouts and ventilation arrangements.
Who is Required to Submit Part O Calculations?
Part O applies to new residential buildings, including houses, flats, student accommodation and care homes. A change-of-use project that creates new homes may also fall within scope. Confirm the project’s classification and evidence requirements with building control. Part O does not apply to extensions or conservatories added to existing buildings.
In England in 2026, Part O compliance is a mandatory requirement for building control approval for new residential buildings within its scope. The 2021 edition remains in force. A full technical review was confirmed in March 2026, but no immediate changes were made. Check the applicable requirements at the outset, especially for a conversion or a project with a complex glazing design. New residential projects in England must also meet Part G water efficiency requirements, and using a water efficiency calculator early can help confirm whether fixture choices will satisfy the 125-litre or 110-litre standard before the design is finalised. If you are unsure which assessments your project requires, an energy assessment checklist for UK property projects can help you identify the right calculations and avoid commissioning a service that does not fit your situation.
The Two Compliance Paths: Simplified Method vs. Dynamic Thermal Modelling
Part O provides two assessment routes. The Approved Document O describes the Simplified Method, a prescriptive check that compares glazing with limits based on floor area, orientation, location risk and cross-ventilation. It may suit a straightforward design that can meet those criteria. In high-risk locations, south-facing glazing may be limited to 11–15% of floor area, depending on cross-ventilation.
High-risk areas include urban and some suburban parts of London, where the Simplified Method may set tighter glazing limits than in moderate-risk areas. For a North or Central London project, establish the site’s risk category and test the proposed layout against the relevant criteria. Cross-ventilation requires habitable rooms to have openings on two different orientations, so check this alongside the room layout, not as a last-minute adjustment.
Dynamic Thermal Modelling (DTM), using CIBSE TM59 methodology, simulates overheating risk for the proposed building. It can assess designs that do not fit the Simplified Method’s prescriptive limits, including schemes with larger glazed areas or single-aspect flats. SAP calculations provide energy-performance information for the wider design, but they do not replace a Part O assessment. Coordinating the two assessments early can help identify design conflicts.
When to Choose Dynamic Thermal Modelling (TM59)
Consider DTM for designs with high glazing ratios, single-aspect flats or external noise that makes open-window ventilation a concern. It may also be suitable for designs using mechanical ventilation with heat recovery (MVHR). Rather than applying fixed glazing limits, modelling tests the proposed design against overheating criteria, which may give more scope to retain the intended architectural design. For urban schemes in London or Milton Keynes, assess the actual site and layout instead of assuming one method will always apply. Coordinating SAP calculations with the overheating assessment can also help manage the design work.
Achieving Compliance: Practical Strategies and Next Steps
Start by considering where unwanted heat enters, then plan how to remove excess heat. External shading can block sunlight before it reaches the glass, while carefully selected glazing can reduce solar gain. The g-value indicates how much solar energy passes through the glass. A lower g-value can limit heat entering while still allowing daylight. Assess glazing as part of the overall Part O strategy, rather than choosing it in isolation.
Next, check whether the ventilation strategy is workable for the site. Open windows may be less dependable in urban locations where noise or security concerns make them impractical. If the design relies on opening windows, consider whether occupants can use them as intended. If not, assess an alternative ventilation strategy. Consider mechanical cooling in the context of the full design. The Approved Document O sets out the compliance guidance. Early part o overheating calculations can flag design issues while there is still time to consider changes.
If an assessment identifies a risk, review the findings in a practical order: check shading and glazing first, then review ventilation openings and cross-ventilation, and reassess the strategy. This helps the team focus on the design factors most relevant to the assessment before construction decisions are difficult to change.
Expert Compliance Support with Energy Smart
Energy Smart provides overheating and SAP calculations, as well as heat loss calculations. Coordinating these assessments can help project teams consider energy performance, heat loss and overheating together. Energy Smart works across Herts, Beds, Cambs, Northants, Bucks and North London, including projects in London, Bedford, Cambridge, Northampton, Luton, Milton Keynes, St Albans, Stevenage, Aylesbury and Peterborough. For a site-specific assessment, share the project details and design information with the team.
Plan a Cooler, Compliant Home from the Start
Part O compliance is easier to plan for as part of the design, rather than as a last-minute check. Choose the assessment route that suits the building, then consider glazing, shading and ventilation together. Early part o overheating calculations can reveal issues while there is still time to assess practical design changes.
Energy Smart provides SAP and overheating assessments for projects across London and the Home Counties. Coordinating the assessments can help your team consider energy performance and overheating requirements together.
To discuss the right assessment for your project, get a quote for Part O calculations. Early assessment can help you address design issues before they become harder to resolve.
Frequently Asked Questions
Is a Part O calculation the same as a SAP calculation?
No. A SAP calculation assesses a dwelling’s energy performance, while a Part O assessment evaluates overheating risk and demonstrates whether the design meets the relevant requirements. The assessments use related design information, so coordinating them can help identify conflicts involving energy performance, glazing and ventilation. Part O overheating calculations are separate and do not replace SAP.
What happens if my building design fails the overheating assessment?
A failed assessment means the current design has not demonstrated compliance. Review the findings with your assessor and consider changes such as reducing solar gain through shading or revised glazing, or adjusting the ventilation strategy. If the Simplified Method’s limits do not suit the design, ask whether dynamic thermal modelling is appropriate. Reassess revised proposals before relying on them for building control approval.
Do I need Part O calculations for a home extension?
Part O does not apply to extensions or conservatories added to existing buildings. It applies to new residential buildings in England, including homes created through some change-of-use projects. If your plans combine an extension with a conversion or new dwelling, check the project’s scope with building control rather than assuming the whole development is exempt.
Why is London treated differently in overheating regulations?
Some urban and suburban parts of London are classed as high risk, reflecting greater overheating risk in those locations. This can make the Simplified Method’s glazing limits more restrictive, but the risk classification depends on the site, not simply the city name. Energy Smart supports projects in London, Bedford, Cambridge, Northampton, Luton, Milton Keynes, St Albans, Stevenage, Aylesbury and Peterborough.