Simplified and dynamic TM59 overheating risk assessments for all new dwellings under Part O (2021). Early engagement identifies design solutions before they become costly changes.
Send us your postcode and project details — we'll confirm which method applies and provide a quote within one working day.
Thanks — we'll confirm the method required and provide a quote within one working day.
Approved Document O came into force in June 2022 as a new requirement of the Building Regulations for England. It introduced, for the first time, a mandatory requirement to assess and control overheating risk in new residential buildings — requiring developers and architects to demonstrate that dwellings are designed to limit the risk of thermal discomfort from solar and internal heat gains before Building Control sign-off.
The regulation recognises that the UK's progressively warmer summers, combined with construction trends towards greater airtightness, higher glazing ratios, and reduced thermal mass, have made overheating a significant risk in modern new-build homes. Without active mitigation, a well-insulated, airtight home can overheat severely during summer months — creating health risks for occupants, particularly the elderly, the very young, and people with medical conditions.
An overheating assessment is required at the design stage as part of the Building Control submission. It cannot be carried out as an afterthought at completion — the assessment drives design decisions around glazing, shading, ventilation, and orientation that must be incorporated before the building is built.
Part O provides two compliance routes. Which one applies to your project depends on the location of the site. The dynamic method requires more detailed modelling but applies to a substantial portion of England.
The Simplified Method uses a prescriptive approach based on limiting solar gain through glazing and ensuring adequate cross-ventilation provision. Rather than modelling internal temperatures directly, it works by controlling the inputs — glazing area, orientation, shading, and ventilation opening sizes — to keep them within limits that Part O considers acceptably low risk.
It is faster and less costly to carry out than TM59, and is appropriate for straightforward dwellings in lower-risk postcode areas. Where it applies, it is often the most cost-effective compliance route — particularly for traditional house types with conventional orientations.
Dynamic Thermal Modelling uses validated building simulation software to model the hour-by-hour internal temperatures of each dwelling throughout a typical summer. The methodology follows CIBSE Technical Memorandum 59, which sets the pass criteria: bedrooms must not exceed 26°C for more than 1% of occupied hours, and living rooms must not exceed 28°C.
It is required for all sites in the postcode zones listed in Appendix C of Part O — which covers significant parts of London, the South East, East Anglia, and the East Midlands where summer overheating risk is elevated. It is also required where the Simplified Method cannot be satisfied, or where the building is more complex.
Approved Document O Appendix C defines a set of postcode districts across England with elevated overheating risk, primarily reflecting the urban heat island effect and local summer climate. For any site in these zones, dynamic TM59 modelling is mandatory regardless of building type or glazing area.
The Appendix C list was updated in April 2025 and now covers a broader range of districts than when the regulation first came into force. Always check the current Appendix C list against your specific postcode before assuming the Simplified Method applies — the boundary of the elevated-risk zone does not always follow intuitive geographic lines.
We check the postcode at the point of enquiry and confirm which method applies before you commit to a fee. There is no charge for this initial check.
Not sure which zone your site is in? Send us your postcode and we'll check the current Appendix C list and confirm immediately — at no charge.
Check my postcode →Overheating is fundamentally a design problem, not a compliance problem. The factors that determine whether a building will overheat — orientation, glazing area and position, shading, ventilation strategy, thermal mass — are all decided early in the design process. Once the design is locked, the options for remediation narrow significantly and become more expensive.
An overheating assessment at RIBA Stage 2 or early Stage 3 gives the design team time to incorporate the necessary measures without redesign cost. Simple adjustments to glazing position, overhang dimensions, or ventilation opening size can resolve an assessment failure that would otherwise require a significant design change.
We are happy to engage as an advisory service at the early design stage — reviewing orientation, glazing ratios, and shading proposals informally before a formal assessment is carried out. This avoids the frustration of a first-pass assessment failure and gives architects the confidence to commit to a glazing strategy knowing it will comply.
We work from early-stage drawings through to the formal assessment. Early informal reviews are available at no extra charge to help the design team make informed decisions before committing to a glazing or shading strategy.
Not every initial assessment passes first time — particularly for highly glazed or densely urban schemes, or for buildings in elevated-risk postcode zones where summer temperatures are already higher. A failure does not mean the design cannot comply; it means the current balance of glazing, shading, and ventilation needs adjustment.
Because TM59 models the building at the level of individual rooms and hours, it identifies exactly which dwellings or rooms are failing and by how much. This precision allows remediation to be targeted — a specific room failing by a small margin usually requires only a modest change to glazing area, shading depth, or ventilation opening size rather than a wholesale redesign.
We include one round of design revision and reassessment within our standard fee. Where larger changes are needed, we advise on the most cost-effective compliance path before any redesign work begins.
Overheating risk is assessed during the design stage, before construction begins. Early engagement means we can flag risks and recommend practical solutions while changes are still affordable.
Overheating risk assessments for new residential developments across Birmingham, Solihull, Coventry, Wolverhampton, and the wider West Midlands. Simplified and dynamic TM59 methods for Approved Document O compliance.
Part O overheating assessments across Bristol, Bath, South Gloucestershire, and the surrounding South West. We understand local authority sustainability policies and their impact on glazing ratios and ventilation strategies.
Based in Worcester — overheating assessments across Malvern, Droitwich, Evesham, Bromsgrove, and Redditch. Our home county with the fastest turnaround and deepest local planning knowledge.
Approved Document O assessments for new homes across Cheltenham, Gloucester, Stroud, Tewkesbury, and the Cotswolds. Early design flags included to avoid costly remedial measures.
Overheating risk modelling for residential projects across Oxford, Banbury, Bicester, Witney, and the wider Oxfordshire area. Dynamic TM59 simulation available for complex or high-risk designs.
As Elmhurst-accredited assessors, our overheating reports are valid across the whole of England and Wales. We regularly assess projects outside our core regions — contact us wherever your site is.
Looking for overheating assessments near you? Whether you're designing a housing scheme in Birmingham, an apartment block in Bristol, or a sensitive conversion in the Cotswolds — our Elmhurst-accredited assessors deliver Approved Document O compliant results with practical design recommendations. Call 01386 365145 or email us to get started.
Part O currently applies to new residential buildings — it does not apply to extensions to existing dwellings or to conversions of non-residential buildings to residential use in the same way it applies to new builds. The regulation is specifically focused on new buildings constructed after June 2022.
However, some local planning authorities have started requiring overheating evidence for permitted development changes and conversions via planning conditions. If you have a condition requiring an overheating assessment for an extension or conversion, we can advise on the most appropriate methodology and carry out the assessment.
CIBSE TM59 sets two criteria for residential overheating assessment. For living rooms, the operative temperature must not exceed 28°C for more than 1% of occupied hours during the calculation period. For bedrooms, the criterion is that the operative temperature must not exceed 26°C for more than 1% of sleeping hours (10pm–7am).
Bedrooms are generally the harder criterion to satisfy because the night-time cooling window is limited and occupants are most sensitive to thermal discomfort when sleeping. This is why south and west-facing bedrooms with significant glazing are the most common failure point in TM59 assessments.
Part O has a preference for passive overheating mitigation — the regulation requires that buildings first demonstrate compliance without mechanical cooling, or where cooling is included, that the cooling demand is minimised through passive design measures.
Where mechanical cooling is installed, Appendix D of Part O requires a check that the cooling energy demand does not exceed a defined limit. This "cooling check" uses the CIBSE TM59 model results to verify that the cooling system is not being relied upon to the extent that it creates excessive energy demand. In practice, this means cooling can assist compliance but cannot be the sole means of achieving it.
As early as possible — ideally at RIBA Stage 2 when building orientation, massing, and glazing strategy are being determined. An informal review at this stage costs very little and can prevent the glazing or shading specification from being locked in at a level that will fail the formal assessment.
The formal assessment for Building Control submission needs to be completed before the design is finalised — because if the assessment identifies changes needed, those changes need to be incorporated into the drawings before they are submitted. An assessment carried out at Stage 4 when drawing packages are complete is significantly harder to act on.
They are separate assessments covering different aspects of building performance. SAP (Part L) assesses energy efficiency and carbon emissions — primarily a winter heating performance concern. Part O specifically addresses summer overheating risk. They are both required for new residential buildings but use different methodologies.
The overglazed extension calculation under Part L is also separate from Part O — it addresses the energy performance implications of excessive glazing in an extension, while Part O addresses the summer comfort implications. Extensions are currently subject to Part L but not Part O, though this can be required by planning conditions.
Approved Document O is specific to England. Wales has its own Building Regulations administered by the Welsh Government, and currently does not have an equivalent mandatory overheating requirement under the Welsh Approved Documents.
However, some Welsh local planning authorities include overheating requirements through planning conditions, and good practice guidance from CIBSE and the UK Green Building Council recommends overheating assessment for new residential buildings in Wales as well. We can carry out TM59 assessments for Welsh projects where required by a planning condition.
Send us your site postcode and we'll confirm which method applies, provide a fee proposal, and advise on any early design considerations — within one working day.
Send us your project details and we'll confirm the method required, provide a fee proposal, and advise on any early design issues — within one working day.
We'll check the postcode, confirm the method, and come back within one working day.