These are alternative assessment approaches within Approved Document O. Our overheating assessment service covers route selection; the TM59 and TM52 service explains the modelling work.
| Decision | Simplified method | Thermal modelling |
|---|---|---|
| Assessment basis | Checks the geometry and opening provision against the guidance applicable to the design. | Calculates thermal behaviour using a model, weather data and defined assumptions. |
| Glazing choices | Requires the proposal to fit the prescribed approach to limiting solar gains. | Allows the effects of glazing, shading and other design measures to be assessed together. |
| Information needed | Accurate room areas, glazing dimensions, orientation, shading and usable opening details. | Those inputs plus fabric, occupancy, gains, ventilation controls and other modelling assumptions. |
| Design feedback | Identifies where a dimension or provision falls outside the simplified guidance. | Shows which assessed spaces fail the selected criteria and supports testing revised options. |
| Practical constraints | Openings still need to be usable in the circumstances assumed by the assessment. | A detailed model also needs realistic opening and ventilation assumptions to be useful. |
| Record to retain | The calculation, supporting drawings and completed compliance information. | The model report, input schedule, results and the measures relied upon in the final design. |
(Source: Approved Document O)
Test the Part O simplified method against the actual openings
A window schedule can look complete while missing the information an overheating assessment needs. Overall frame dimensions do not establish how much air can pass through an opening. Ask the designer to identify the opening arrangement, restrictors, fixed panes and any surrounding detail that affects use. Tie each entry to a room and drawing reference so an assessor can follow the evidence without guessing.
For the simplified route, Approved Document O distinguishes location risk and cross-ventilation when setting its approach. Do not infer cross-ventilation merely because a dwelling has windows on different elevations. The proposed air path and room arrangement need checking against the document. A layout change can therefore alter the assessment even when the total glazing remains unchanged.
Use a marked-up plan during the design review. Record which openings provide heat removal, which are fixed and which depend on doors remaining open. This creates a useful conversation between the architect, window supplier and assessor before an attractive elevation becomes a procurement commitment.
When the Part O simplified method stops being a useful design route
Consider a dwelling with generous glazing and fixed architectural shading. First establish whether the simplified checks can represent the proposal and whether sensible amendments would bring it within that route. If the design team wants to keep features that the prescribed checks cannot accommodate, ask for a modelling scope rather than repeatedly adjusting isolated dimensions without testing the whole design.
The modelling route in Approved Document O uses TM59 with the document's own provisions. A model is an assessment method, rather than permission to retain any amount of glass. It may still identify a need for shading, different glazing or a revised ventilation strategy. Read TM52 vs TM59 before specifying the assessment methodology.
Agree the question before commissioning the model. Testing a preferred design differs from comparing several design options. Identify the drawings that form the base case, the alternatives to examine and who will approve a change. This avoids a report describing a superseded arrangement while the construction team orders something else.
Keep Part O simplified checks connected to noise and ventilation
Approved Document O addresses whether heat-removal measures can be used, including constraints involving noise, security, pollution and protection from falling. A favourable calculation cannot settle those practical issues by itself. Flag affected elevations and rooms early, and have the relevant designers explain how their proposals work together.
Background ventilation and summer heat removal answer different design questions. The ventilation guidance in Approved Document F should be coordinated with the overheating strategy, rather than treated as evidence that the heat-removal assessment has already been completed. Our Part F ventilation certificate guide explains the separate commissioning evidence.
For a self-build, keep a single schedule of the glazing and ventilation decisions shared with suppliers. On a larger development, identify dwelling variants rather than assuming a repeated floor plan has the same exposure and opening constraints everywhere. The new-build compliance package helps place this work alongside the other project evidence.
Freeze the Part O simplified method evidence before ordering
Before purchasing windows, compare the assessed schedule with the supplier's proposed products. Check opening operation as well as appearance. If a substitution changes the usable opening, shading or glazing specification, send the detail back for review. Record the assessor's response with the revised drawing instead of relying on a verbal assurance that the replacement is equivalent.
Send plans, elevations, opening details and known site constraints to our overheating assessment service. Include any planning condition separately because planning requirements differ by council, as explained by planning practice guidance on making an application. Ask the quote to identify the proposed assessment route, information gaps and responsibility for reviewing later design changes.
About our Overheating Assessments service
For the service itself, see Overheating Assessments.