Simplified Overheating Assessments
Frequently Asked Questions
In simple terms, it's a quick, rule-based check to confirm a new dwelling won't overheat in summer, without needing full computer modelling.
It looks at things like window size, orientation and ventilation, and compares them against fixed limits set out in Building Regulations Part O. If your design sits within those limits, that's your compliance route sorted — no dynamic modelling required.
The simplified method is a fixed set of pass/fail rules; TM59 is a full dynamic thermal simulation.
TM59 gives a much more precise, design-specific answer and is more forgiving of unusual layouts or heavily glazed designs — though naturally, that extra precision takes more time and cost to produce than a simplified check.
We always strive to deliver the most cost-effective solution for your project, and will never charge you for a more detailed assessment than you actually need — if your design comfortably fits the simplified method's criteria, that's the route we'll recommend.
If you're building a new dwelling without mechanical cooling, yes, it's a mandatory Part O requirement.
If you're not sure whether your project counts, get in touch and we'll confirm.
Not every project qualifies.
Larger amounts of glazing, certain ground-floor or easily-reachable bedroom windows (which building control may require to be treated as closed at night for security), and homes in higher-risk locations can all push a project into needing full TM59 modelling instead.
The safest approach is to check early — ideally before window sizes and locations are finalised — so the design can still be adjusted if needed, rather than forcing an expensive redesign later.
It's essentially a measure of openable window area — passive ventilation — expressed as a percentage of floor area, checked both for the building as a whole and room by room.
How much opening area you need depends on the building's orientation and which direction the most heavily glazed areas face.
What it doesn't account for is things like the g-value of the glazing, the U-values of the building fabric, or shading from nearby buildings — which is exactly why a full dynamic (TM59) assessment can sometimes pass a design that the simplified method would fail on paper.
To carry out a simplified overheating assessment, we'll need:
- To-scale elevations and floor plans, showing which windows are openable and roughly how far they open
- A site plan, or clear indication of the building's orientation
- The site address
If you're not sure what you've got available, get in touch and we'll talk through what's needed for your project.
That's not a dead end — it just means the project moves to full TM59 dynamic modelling instead.
TM59 is more detailed and better able to account for design features the simplified method can't, so a "fail" on the simplified check often still results in a straightforward pass once modelled properly. Sometimes, even a small design tweak is enough to bring a project back within the simplified method's criteria and avoid the extra cost of TM59 altogether.
As standard, we'll always talk you through the best next step for your specific project — get in touch and we'll help you find the most cost-effective way forward.
With our current workflow, we can turn around simplified overheating assessments quickly, and we price them competitively from the outset.
Current pricing and lead times are available via our online quoting tool. Alternatively, do get in touch via phone or email if you have questions and would like to confirm details, and we will happily work out how we can best help you.
| Misconception | Response |
|---|---|
| "My house doesn't have much glazing, so I definitely won't need TM59." | Not necessarily — glazing area is only one factor. Ground-floor or easily-accessible bedroom windows that have to be treated as closed at night for security, or being in a higher-risk location, can force TM59 even on a modest design. |
| "Isn't this just more red tape getting in the way of my project?" | We understand it can feel like one more hurdle on top of everything else you're juggling — that's a completely fair reaction. Our job is to make this part as painless as possible, so it doesn't slow your project down. And at the end of it, you'll have a home that's genuinely comfortable to live in through the warmer months for years to come — something you can feel good about, not just a certificate in a drawer. |