If you are developing new residential property in England or Wales, Part O overheating compliance is now one of the most scrutinised elements of the Building Regulations sign-off process. What began in 2022 as a relatively straightforward check on window sizes has evolved into a more demanding assessment, particularly for high-density urban schemes. This guide explains how Dynamic Thermal Modelling using IES VE software provides the clearest path to compliance, helping you avoid the redesign costs and Building Control rejections that are increasingly common across the sector.
Table of Contents
- What Is Part O and Why Does It Matter in 2026?
- Simplified Method vs. Dynamic Thermal Modelling: Which Route Is Best?
- Key Parameters IES Modelling Tests for Part O Compliance
- Step-by-Step: How CCA Environmental Runs a Part O IES Assessment
- Common Part O Compliance Failures and How to Fix Them
- The Cost of Non-Compliance: Why Early IES Modelling Saves Money
- Frequently Asked Questions About Part O Overheating
- Conclusion: Future-Proof Your Development with IES Modelling
What Is Part O and Why Does It Matter in 2026?
Part O of the Building Regulations, introduced under Requirement O1, came into force on 15 June 2022 in England and 23 November 2022 in Wales. Its purpose is simple but far-reaching: new residential buildings must be designed to limit unwanted solar gains and to remove excess heat through passive means. The regulation covers all new dwellings, including flats, student accommodation, and care homes, though hotels remain outside its scope.
The stakes are high. If a development fails to demonstrate Part O compliance, Building Control cannot issue a completion certificate. Without that certificate, the property cannot be sold, rented, or legally occupied. Developers who treat overheating as a late-stage box-tick exercise are finding themselves exposed to costly delays.
Two compliance routes are available. The Simplified Method uses prescriptive tables comparing glazed area to floor area, while Dynamic Thermal Modelling, or DTM, uses software such as IES VE to simulate internal temperatures across a full year using location-specific weather data. In 2026, with the Building Safety Regulator keeping Approved Documents under active review and climate projections pointing to more frequent and intense heatwaves, the Simplified Method is becoming harder to rely on for anything beyond the most straightforward designs. DTM has quietly become the default for schemes where glazing ratios, orientation, or ventilation strategies fall outside the prescriptive limits.
Simplified Method vs. Dynamic Thermal Modelling: Which Route Is Best?
When the Simplified Method Works
The Simplified Method suits low-risk designs: detached or semi-detached houses in moderate-risk locations, with good cross ventilation and conservative glazing ratios. It works from fixed tables. For example, in high-risk urban and suburban London postcodes, a south-facing room with cross ventilation is limited to a total glazed area of 15 percent of the floor area, with a maximum of 22 percent in the most glazed room. In moderate-risk locations, those figures rise to 15 percent overall and 30 percent in the most glazed room.
Without cross ventilation, the limits tighten sharply. South-facing rooms in high-risk areas drop to just 11 percent overall and 11 percent in the most glazed room. If your design sits within these boundaries and has straightforward orientation, the Simplified Method is quick and requires no specialist software. The moment you exceed any limit, however, you face a choice: reduce window sizes, add external shading, or switch to DTM.
Why IES Modelling Gives You More Design Freedom
Dynamic Thermal Modelling does not rely on fixed ratios. Instead, it simulates how a specific building will perform thermally, using CIBSE weather files that now include future climate scenarios for 2026 and beyond. This means you can specify larger glazed areas, more complex orientations, and innovative shading strategies, provided the model demonstrates that internal temperatures stay below the overheating threshold. For bedrooms, that threshold is typically 26 degrees Celsius for no more than one percent of occupied hours.
IES VE is the industry-standard platform for this work. It models solar gain, natural ventilation, thermal mass, and any mechanical cooling within a single integrated environment. For any development in a high-risk London location, or any scheme with non-standard floorplans, single-aspect flats, or significant west-facing glazing, commissioning IES modelling at the design stage is the most reliable way to secure compliance without compromising the architectural vision. The cost of modelling is modest compared to the expense of redesigning glazing and shading systems after planning has been secured.
Key Parameters IES Modelling Tests for Part O Compliance
Solar Gain Limitation: G-Values and Glazing
The model examines the G-value, or solar heat gain coefficient, of every glazed element. A lower G-value, typically 0.4 or below, reduces the amount of solar radiation entering the room, directly lowering overheating risk. The trade-off is that very low G-values can reduce daylight transmission, so the model helps strike a balance that satisfies both thermal comfort and internal amenity.
Orientation is critical. South and west-facing façades receive the most intense solar exposure during summer afternoons and evenings, precisely when internal temperatures are already elevated. IES simulates the sun’s path across the entire year, calculating peak internal temperatures hour by hour. External shading devices, including louvres, brise-soleil, and overhangs, are modelled explicitly. Internal blinds, by contrast, are not recognised as effective mitigation under Part O, because they allow solar radiation to enter the room before blocking it, trapping heat between the blind and the glass.
Cross Ventilation and Airflow Rates
Adequate cross ventilation, defined as openings on opposite sides of a dwelling, is the most effective passive method for purging excess heat. The IES model simulates wind pressure coefficients and opening sizes, including windows, doors, and trickle vents, to calculate air changes per hour across every thermal zone.
Where cross ventilation is not possible, typically in single-aspect flats, the glazing limits under the Simplified Method become highly restrictive. DTM allows you to test alternative strategies. Mechanical ventilation systems, enhanced passive stack ventilation, or a combination of both can be modelled to demonstrate that adequate heat removal is achieved. The software flags any zones where the ventilation rate falls short, giving the design team a clear target for improvement.
The Part L Tension: Energy Efficiency vs. Overheating
A tension that runs through every Part O assessment is the conflict with Part L, which governs the conservation of fuel and power. Part L demands high levels of insulation and airtightness to minimise heat loss in winter. Those same properties trap heat in summer, making overheating more likely. This is not a theoretical problem: it is the most common underlying cause of compliance failures in 2026.
IES modelling resolves this by running both assessments in parallel. The same thermal model can be used for Part L calculations via SAP or SBEM, and for Part O overheating analysis using the CIBSE TM59 methodology. This integrated approach allows the design team to test combinations of high-performance glazing, such as triple-glazed units with selective coatings that block solar gain while retaining a high U-value for winter insulation. A critical warning: relying on air conditioning to solve overheating can cause a Part L failure, because the energy consumption of the cooling system undermines the dwelling’s overall efficiency rating. The model must demonstrate that passive measures have been prioritised before any mechanical cooling is considered.
Step-by-Step: How CCA Environmental Runs a Part O IES Assessment
The process begins with data collection. We gather architectural drawings, glazing specifications, shading details, and the site postcode, which determines whether the location is classified as high or moderate risk under Approved Document O. This postcode check is essential because the weather file and compliance thresholds differ between risk categories.
Next comes the model build. The building is constructed in IES VE with all thermal zones defined, construction materials assigned, and HVAC systems specified. We follow the CIBSE TM59 methodology for residential overheating, which sets out the simulation protocols, occupancy profiles, and assessment criteria that Building Control expects to see.
The simulation itself runs a full year of hourly weather data. For 2026 projects, we use the current CIBSE future weather file, which incorporates climate projections reflecting hotter and more prolonged summer conditions. The software checks for exceedance of the threshold: for bedrooms, no more than one percent of occupied hours above 26 degrees Celsius; for living areas, the limit is 28 degrees Celsius.
We then produce a compliance report with clear pass or fail results for every unit and every thermal zone. Where a design fails, the report includes specific, actionable recommendations: add external shading to a particular façade, increase the opening area of windows in a named room, or switch to a lower G-value glazing specification. The final step is iteration. We work with the design team to adjust parameters and re-run the model until compliance is achieved. This iterative loop, completed during the design stage, is far faster and cheaper than redesigning elements post-planning or, worse, during construction.
Common Part O Compliance Failures and How to Fix Them
The first common failure is over-glazed south-facing façades in high-risk areas. The fix is either to reduce window area, add fixed external louvres, or switch to DTM to prove that the design works despite exceeding the Simplified Method limits.
The second is single-aspect flats with no cross ventilation. Here, the solution typically involves mechanical ventilation with heat recovery, or MVHR, or passive stack ventilation. The IES model quantifies the required airflow rate, allowing the services engineer to specify equipment with confidence.
The third failure is the conflict with Part L. A design that insulates well for winter may overheat in summer. The fix is to use dynamic glazing or high-performance triple glazing with a low G-value but high U-value, and to test both regulations simultaneously in the IES model.
The fourth failure is underestimating future climate. A design that passes using historical weather data may fail when tested against the 2026 or 2030 projections. Building Control is increasingly asking for future-climate evidence. Using the appropriate weather file in IES from the outset prevents a pass today becoming a fail at completion.
The Cost of Non-Compliance: Why Early IES Modelling Saves Money
When a development fails Part O at the Building Control stage, the entire project can stall. Redesigning windows, adding structural shading, or retrofitting mechanical ventilation mid-construction is significantly more expensive than modelling the building correctly at the design stage. A full IES assessment for a typical 50-unit apartment block costs in the region of £2,000 to £5,000. A single redesign of glazing and structural shading, by contrast, can exceed £20,000 in architectural fees and material changes alone, before accounting for programme delays.
While no official statistics on Part O failure rates have been published, anecdotal evidence from the 2024 to 2025 period suggests that roughly one in five new-build schemes initially fails due to over-glazing or poor ventilation strategy. Early IES modelling eliminates this risk by identifying problems when they are still inexpensive to fix.
Frequently Asked Questions About Part O Overheating
Does Part O apply to existing buildings? Not currently. The regulation applies only to new-build dwellings. However, the government is consulting on extending the scope to include Material Change of Use conversions, such as office-to-residential schemes. If you are undertaking a conversion project in 2026, check with your local authority, as requirements may be applied through planning conditions even before formal regulatory change.
Can I use air conditioning to comply? Yes, but it must be modelled in IES and must not cause a Part L failure. Passive measures, including shading and natural ventilation, must be demonstrated as the primary strategy before mechanical cooling is introduced.
What is the difference between Part O in England and Wales? The technical requirements are nearly identical, but Wales has its own Approved Document, effective from 23 November 2022, with separate transitional arrangements. Always check the devolved regulation for your project location.
Do replacement windows in an existing home need to comply? No. Part O applies only to new-build dwellings. Replacement windows in existing homes are covered by Part L for thermal performance and by the general Building Regulations for safety and ventilation.
Conclusion: Future-Proof Your Development with IES Modelling
Part O is not a temporary requirement. With the Building Safety Regulator reviewing Approved Documents and climate projections pointing to hotter UK summers, compliance will only become more stringent over the coming years. The Simplified Method remains a useful starting point for straightforward, low-risk designs, but for complex, high-density, or high-risk developments, Dynamic Thermal Modelling using IES VE is the only reliable path to a pass. Early modelling protects your programme, your budget, and your design intent.
Contact CCA Environmental today for a free initial consultation on your Part O overheating strategy. We will review your drawings and advise on the most cost-effective route to compliance, giving you certainty before you submit for Building Control approval.