A Simplified Overheating Assessment is a compliance method allowed under Approved Document O (Overheating), 2022 edition, for domestic buildings such as new build houses and flats. Instead of carrying out a full dynamic simulation under TM59, the simplified route allows design teams to demonstrate acceptable overheating risk using fixed design limits based on glazing area, orientation, and openable areas of windows.
This method is often used during early-stage design or planning submissions where a quick compliance confirmation is needed without committing to full simulation costs. However, it only applies if the dwelling meets certain criteria set by the government. If the property exceeds glazing or ventilation thresholds or falls into a high-risk category such as London or urban heat island zones, the simplified route is no longer valid, and a TM59 dynamic modelling report becomes mandatory.
How the Simplified Tool Works
The official tool used for this method follows a step-by-step pathway:
Risk Checker
First, the building is assessed against location, cross-ventilation and occupancy type. If it falls into a high-risk category, it moves out of the simplified route.
Cross-Ventilation vs Non-Cross-Ventilation Pathway
The tool separates homes with good airflow potential from those that only have single-sided ventilation. Each path has different glazing and free area limits.
Glazing Area Limits
The total glazing in living rooms, kitchens and bedrooms is compared against maximum permissible percentages of floor area. South and west-facing elevations are treated more strictly due to solar gain.
Free Area (Ventilation Opening) Check
The tool calculates the minimum equivalent openable area required through windows or vents. This ensures that, in hot weather, the dwelling can purge heat naturally.
Pass or Escalate
If both glazing and free area requirements are met, the Simplified Overheating Assessment is acceptable for Building Control or planning sign-off. If not, TM59 dynamic modelling is required.
When a Simplified Assessment Is Suitable
Based on our experience delivering Part O reports, this method works best for:
- Standard new-build houses
- Low risk dwellings with cross-ventilation
- Developments outside high-risk urban heat zones
- Designs with modest glazing proportions and operable windows
It becomes unsuitable where large bi-folds, floor-to-ceiling glazing, single-sided flats or high urban density are involved. In those cases, the tool will automatically redirect the assessment to TM59.
How a Simplified Assessment Supports Design Teams
Design teams often use the Simplified Assessment at concept stage to confirm whether planned glazing sizes and ventilation strategies are viable before final planning drawings are issued. It allows architects to adjust opening sizes and orientation while changes are still easy to action. Builders also find value in receiving a clear schedule of minimum free area requirements, as this avoids later redesign of window specifications.
Because the assessment follows a rigid structure, it provides a straightforward compliance path, but it also makes non-compliance very clear. That transparency helps prevent delays during Building Control review, as the evidence matches the official government tool exactly.
Why Early Assessment Prevents Costly Redesign
We frequently receive instructions when drawings are already fixed, and planning has been submitted. At that point, even a marginal glazing overshoot can trigger TM59 modelling, which is significantly more complex and time intensive. By carrying out a Simplified Assessment early, the design team can either confirm that the simplified method is viable or identify straight away that dynamic simulation will be required.
This early clarity avoids additional consultant fees, redesign time and delays in technical approval.
Commissioning a Simplified Overheating Assessment
At Energy Digest, we carry out Simplified Overheating Assessments using the official government-approved calculation method, producing clear documentation ready for submission to planning or Building Control. If the dwelling passes the simplified criteria, we provide a formal compliance statement with ventilation free area tables and glazing percentage confirmation. If the simplified route is not available, we clearly set out the next steps and can seamlessly progress to TM59 modelling to keep the project moving without interruption.
A common concern clients raise is the cost risk, specifically, “If the design fails the Simplified Overheating Assessment and I then need a TM59 Assessment, will I have to pay twice?” At Energy Digest, we remove that uncertainty. If the dwelling does not pass the simplified method, we waive the simplified assessment fee entirely and proceed straight to the TM59 dynamic simulation, only charging for the TM59 service. This means you can begin with the most cost-effective route without worrying about duplicated fees or wasted spend.
