A practical guide for homeowners, architects, builders and property developers
JPEC Building Services provides professional Part O overheating assessments and TM59 dynamic thermal modelling across Horley, Reigate, Redhill, Crawley, Surrey, West Sussex, East Sussex, Kent and the wider South East.
Modern homes are designed to retain heat and use less energy, but highly insulated and airtight properties can also become uncomfortably warm during the summer.
Large windows, limited shading, restricted ventilation and high levels of solar gain can all increase the risk of overheating, particularly in bedrooms, apartments and properties with south- or west-facing glazing.
A Part O assessment can help demonstrate that a new residential building has been designed to limit unwanted solar gains, provide an effective way for excess heat to escape, maintain suitable internal temperatures, protect occupant comfort and safety, and provide appropriate evidence to Building Control.
What Is Building Regulations Part O?
Part O is the section of the Building Regulations in England that addresses overheating in new residential buildings. Its purpose is to reduce the risk of new homes becoming excessively hot during warmer weather.
The design must make reasonable provision to limit unwanted solar gains during the summer and provide an adequate method of removing excess heat from inside the building. The assessment must also consider whether the proposed measures are safe, practical and usable by occupants.
- Window sizes and orientation
- Glazing performance and solar gain
- External and internal shading
- Window-opening areas and cross-ventilation
- Noise, security and pollution
- Occupant safety and window restrictors
- Mechanical ventilation or cooling where necessary
Does My New-Build Home Need a Part O Assessment?
New residential buildings in England are generally required to address the overheating requirements of Building Regulations Part O.
- New-build houses
- New flats and apartments
- New student accommodation
- New residential care homes
- New shared residential accommodation
- Other new residential buildings where people sleep overnight
Part O is especially important where a development includes large areas of glazing, south- or west-facing windows, floor-to-ceiling glazing, single-aspect apartments, restricted window openings, high insulation and airtightness, roof-space bedrooms, urban noise constraints or communal heating pipework.
The assessment should be considered while the architectural design can still be changed. Leaving it until construction has started can make compliance more difficult and more expensive.
Does Part O Apply to House Extensions?
Part O does not currently apply to an extension or conservatory added to an existing residential building after that building has been completed. However, overheating should still be considered for highly glazed extensions.
Large bifold doors, roof lanterns, rooflights and south-facing glazing can significantly increase summer temperatures. An overheating review can help identify whether the design may need reduced glazing, solar-control glass, external shading, roof overhangs, improved ventilation or changes to the size and orientation of rooflights.
Does Part O Apply to Conversions?
Part O does not generally apply to a material change of use under its current scope. This can include some projects where an existing commercial, agricultural or other building is converted into residential accommodation.
Conversions can still experience serious overheating problems. Offices converted into flats, shops converted into homes, barn conversions and industrial buildings converted into apartments may have large glazed elevations, limited opening windows or deep floor plans that were not originally designed for residential use.
Why Are New Homes at Risk of Overheating?
Large Windows and Solar Gain
Large windows allow sunlight to enter the building. Solar energy can heat floors, walls, furniture and internal surfaces, particularly through south- and west-facing glazing.
Limited Window Openings
Fixed glazing, restricted openings and small openable areas can prevent heat from escaping. This is particularly important in bedrooms and upper-floor apartments.
Single-Aspect Properties
A single-aspect flat has windows on only one external elevation, making cross-ventilation more difficult.
Noise, Pollution and Security
A design may appear to rely on open windows, but occupants may not be able to use them beside a busy road, railway, airport or commercial site. Ground-floor windows may also be unsuitable for overnight ventilation.
Internal Heat Gains
People, appliances, lighting, hot-water cylinders and communal heating pipework all add heat inside a building.
What Is Assessed During a Part O Overheating Assessment?
The assessor reviews the architectural design, proposed glazing, ventilation strategy and relevant site constraints.
- Floor plans, elevations and building sections
- Site location and property orientation
- Window dimensions and openable areas
- Glazing specifications and solar-gain values
- Window restrictors and opening angles
- External shading, balconies and overhangs
- Mechanical ventilation details
- Surrounding buildings and site obstructions
- Noise, air quality and security constraints
- Occupancy, heating and hot-water information
What Are the Two Part O Assessment Methods?
There are two principal routes commonly used to assess overheating under Approved Document O: the simplified method and dynamic thermal modelling.
The Part O Simplified Method
The simplified method uses prescriptive limits and design checks for glazing area, orientation, solar gains, window openings, ventilation, shading and site location. It can suit straightforward houses and developments but may be restrictive for highly glazed or complex designs.
TM59 Dynamic Thermal Modelling
TM59 is a methodology produced by the Chartered Institution of Building Services Engineers for assessing overheating risk in homes. Specialist software simulates how internal temperatures may change hour by hour.
- External weather conditions
- Solar gains and building orientation
- Window sizes and glazing performance
- Thermal mass, insulation and airtightness
- Occupancy and internal heat gains
- Window-opening patterns
- Mechanical ventilation and shading
- Room layouts and individual room performance
When Is TM59 Modelling Appropriate?
- The simplified method cannot demonstrate compliance
- The building has large areas of glazing
- The project includes flats or apartments
- The dwelling is single aspect
- Window openings are restricted
- Noise, air quality or security limits natural ventilation
- The architectural design is complex
- The developer wants to compare design options
- Mechanical cooling is being considered
- Building Control requests detailed evidence
What Is the Difference Between Part O and TM59?
Part O is a Building Regulations requirement. TM59 is a technical methodology that may be used to assess overheating risk in residential buildings. A Part O assessment may follow either the simplified method or dynamic thermal modelling based on TM59.
Can Opening Windows Prevent Overheating?
Opening windows can remove excess heat, but the solution must be realistic and safe. The assessor may need to consider external noise, traffic pollution, security, fall risk, restrictors, occupant safety, weather exposure and accessibility.
A bedroom may appear to pass if a window is assumed to remain fully open overnight, but that assumption may not be appropriate for a ground-floor bedroom beside a busy road.
How Can External Noise Affect a Part O Assessment?
Developments near Gatwick Airport, busy roads, railway lines, industrial sites, town centres or entertainment venues may not be able to rely on windows being fully open for long periods.
This is particularly relevant in and around Horley, Gatwick, Crawley, Reigate and Redhill. The Part O assessment should therefore be coordinated with the acoustic strategy at an early stage.
How Can Overheating Risk Be Reduced?
- Reduce unnecessary glazing
- Reposition vulnerable windows
- Use appropriate solar-control glazing
- Add external shading, balconies or overhangs
- Install brise soleil
- Provide opening windows on more than one elevation
- Increase safe openable areas
- Improve cross-ventilation
- Use suitable mechanical ventilation
- Reduce exposed communal heating pipework
- Review room layouts
- Improve secure nighttime ventilation
- Use mechanical cooling only where necessary
The most effective approach is generally to limit unwanted heat before trying to remove it. External shading can prevent solar energy entering through the glass, while internal blinds act after much of that energy has already entered.
Can Solar-Control Glass Help?
Solar-control glazing can reduce the amount of solar energy entering a building and may help where windows face south or west, the building has large glazed elevations, or external shading is difficult.
The specification must be selected carefully because it can affect natural daylight, the appearance of the glass, winter solar gains, Part L calculations and heating demand.
Can Air Conditioning Be Used to Pass Part O?
Mechanical cooling may be considered where passive and other suitable measures cannot provide sufficient heat removal, but it should not normally be the first response.
The design should first consider reducing solar gains, providing shading, improving natural or mechanical ventilation, changing glazing specifications and adjusting the layout. Where cooling is included, it must also be reflected in the project energy calculations.
Does Part O Affect SAP Calculations?
Part O and SAP calculations assess different aspects of a dwelling, but design decisions can affect both. SAP assesses regulated energy performance and Part L compliance, while Part O assesses overheating risk.
- Reducing glazing may reduce both heat loss and solar gain
- Solar-control glass may reduce summer gains but also affect winter gains
- Mechanical cooling may increase calculated energy use
- Window-opening changes may affect the ventilation strategy
- External shading may influence daylight and solar performance
JPEC Building Services can support both SAP and Part O assessments, helping the design team consider the building as one coordinated system.
When Should a Part O Assessment Be Completed?
A Part O assessment should ideally be completed during the early design stage, while the architect can still adjust window sizes, window positions, room layouts, shading, balconies, glazing specifications, opening areas and ventilation systems.
Initial Design Review
The drawings are reviewed and obvious overheating risks are identified.
Preliminary Assessment
The building is assessed using the most appropriate Part O method.
Design Development
Potential failures are discussed with the architect, developer and building-services team.
Revised Assessment
The model or calculation is updated following design changes.
Final Compliance Report
The final design is documented for submission to Building Control.
What Happens if a New Build Fails?
A failed assessment means that one or more parts of the proposed design do not satisfy the relevant overheating criteria. The assessor can investigate the causes and model possible improvements.
- Reduce glazing
- Add external shading
- Improve the glazing specification
- Increase openable window area
- Change window restrictors
- Improve cross-ventilation
- Revise room layouts
- Add mechanical ventilation
- Reduce internal heat gains
- Review communal pipework
- Add mechanical cooling only as a final measure
Common Part O Assessment Mistakes
Completing the Assessment Too Late
Once windows and glazing have been ordered, the available design options become more limited.
Providing Inaccurate Window Information
The overall window size is not the same as the openable area. Frames, restrictors and opening angles must be represented correctly.
Ignoring Noise or Security
The assessment should not rely on fully open windows where external noise or ground-floor security makes this unrealistic.
Changing Glazing Without Updating the Assessment
A replacement product may have a different solar-gain value.
Failing to Coordinate Part O and Part L
Changes made for overheating compliance can affect the SAP result and wider energy strategy.
Who Needs a Part O Overheating Assessment?
- Architects
- Property developers
- Housebuilders
- Building contractors
- Planning consultants
- Housing associations
- Local authorities
- Residential care providers
- Student accommodation developers
- Self-build homeowners
- Building Control teams
Part O Assessments for Architects
Early overheating advice helps architects retain more control over the appearance and function of the building. Different window sizes, glazing specifications, shading depths, balcony designs, opening arrangements and ventilation options can be tested while drawings are still being developed.
Part O Assessments for Property Developers
A repeated overheating issue across several plots or apartments can significantly affect cost and programme. Preliminary assessments can help establish a suitable strategy before the design is finalised.
- Individual new-build homes
- Small residential developments
- Apartment buildings
- Student accommodation
- Residential care projects
- Mixed-use developments containing new residential units
- Larger multi-plot schemes
Part O Overheating Assessments Across Surrey, Sussex and Kent
JPEC Building Services provides Part O assessments, TM59 dynamic thermal modelling and building energy-performance services across the South East.
- Horley
- Reigate
- Redhill
- Crawley
- Gatwick
- Dorking
- East Grinstead
- Horsham
- Epsom
- Leatherhead
- Caterham
- Oxted
- Surrey
- West Sussex
- East Sussex
- Kent
- South London
- The wider South East
Many assessments can be completed using architectural drawings and technical project information, allowing suitable developments to be supported remotely across the wider region.
Part O Overheating Assessment Near Me
Working with a local Part O assessor can help communication between the client, architect, developer and Building Control team. JPEC Building Services understands local constraints such as aircraft noise near Gatwick, busy roads, railway noise, urban air quality, single-aspect apartments and highly glazed homes.
How JPEC Building Services Can Help
JPEC Building Services supports homeowners, architects, builders, property developers and commercial clients with building compliance, energy-performance and sustainability services.
- Part O overheating assessments
- TM59 dynamic thermal modelling
- SAP calculations
- Predicted Energy Assessments
- New-build EPC support
- Heat-loss calculations
- Energy Performance Certificates
- Roof surveys
- Renewable-energy suitability assessments
- Sustainability reports
- Decarbonisation reports
- Building compliance support
We provide practical advice designed to help project teams understand why a building may be at risk and what changes could improve its performance. As part of the wider JPEC Group, we can also connect compliance assessments with electrical, renewable-energy and building-services design where appropriate.
To discuss a Part O overheating assessment or TM59 assessment for a new-build development in Horley, Reigate, Redhill, Crawley, Surrey, Sussex, Kent or the wider South East, contact JPEC Building Services.
Telephone
0800 955 2821
Website
jpecbuildingservices.co.uk
info@jpecgroup.co.uk
Frequently Asked Questions
What is a Part O overheating assessment?
A Part O assessment evaluates whether a new residential building has suitable measures to limit unwanted summer solar gains and remove excess heat.
Does every new-build house need a Part O assessment?
New residential buildings in England generally need to demonstrate compliance with the overheating requirements of Building Regulations Part O.
Does Part O apply to house extensions?
Part O does not currently apply to extensions added to existing residential buildings. However, overheating analysis may still be beneficial for highly glazed extensions.
Does Part O apply to barn conversions?
Part O does not generally apply to a material change of use under its current scope, although an overheating assessment may still be advisable.
What is the difference between Part O and TM59?
Part O is a Building Regulations requirement. TM59 is a detailed methodology that can be used for dynamic thermal modelling of overheating risk in homes.
Can opening windows make a property comply?
Opening windows may form part of the strategy, but noise, security, pollution, restrictors and occupant safety must be considered.
Can air conditioning be used for Part O compliance?
Mechanical cooling may be used where other suitable methods cannot remove enough heat, but it should not normally be the first design response.
When should the assessment be completed?
The assessment should ideally begin during the early design stage, before windows, glazing and ventilation systems are finalised.
Does JPEC Building Services provide Part O assessments in Surrey?
Yes. JPEC Building Services provides Part O overheating assessments and TM59 modelling across Surrey, Sussex, Kent and the wider South East.
This article provides general information only. The assessment method, compliance evidence and design requirements will depend on the individual building, its location, the applicable Building Regulations and the requirements of the relevant Building Control body.
