Overheating Assessments
An overheating assessment checks that a new or converted home won't get dangerously hot, as required by Part O of the Building Regulations. AVA Home Design carries out CIBSE TM59 and TM52 overheating assessments in-house across the whole UK (by the Simplified Method or full Dynamic Thermal Modelling) alongside the design, so your scheme meets Approved Document O without a separate consultant.
Since 15 June 2022, Approved Document O of the Building Regulations requires most new homes to demonstrate that they limit unwanted solar gain and can remove excess heat. For many developers and self-builders this is a new hurdle, and it is often discovered late, after the windows and layout are already fixed.
We assess overheating in-house, at the design stage, as part of the architectural design, not as a separate report bolted on at the end. Because the assessment is a calculation based on your drawings and location, we provide it anywhere in the UK without a site visit. Having the assessment and the design in the same hands is the point: when a result fails, the person who has to change the glazing is already holding the drawing.
Two things about Part O catch people out. It applies to new residential buildings only, but that scope is wider than most expect. It takes in dwellings and flats, care homes and schools where people sleep, accommodation for under-fives, student halls and accommodation for children aged five and over. And requirement O1(2)(b) says mechanical cooling may only be used where insufficient heat can be removed without it. You cannot design in air conditioning and call the problem solved; the passive strategy has to be shown to fall short first.
The two compliance routes, and the two standards
- CIBSE TM59 (dynamic)
- The recognised dynamic-modelling method for homes. It simulates the building hour by hour across a design summer against defined comfort criteria for living spaces and bedrooms.
- CIBSE TM52
- The adaptive-comfort criteria used for non-domestic and communal spaces, applied where TM59 is not the right fit.
- Simplified Method (Section 1)
- Caps maximum glazing area and sets minimum free areas for ventilation, read off tables. Which table applies depends on your location risk category and whether the home has cross-ventilation, meaning openings on opposite façades. Quicker, cheaper, and considerably more restrictive on glazing.
- Dynamic Thermal Modelling (Section 2)
- An hour-by-hour simulation following CIBSE TM59, with Part O adding its own limits on the modeller's choices. More work, but usually the only route that keeps large glazing, and the right choice for a highly shaded site or a location the two standard categories represent badly.
Who this is for
- Developers and self-builders needing Part O sign-off for new dwellings
- Anyone converting a building or changing its use to create new homes
- Design practices that need the overheating report produced for their scheme
Frequently asked questions
- When do I need an overheating assessment?
- Since 15 June 2022, Approved Document O requires new residential buildings (new houses and flats, and many conversions and changes of use that create dwellings) to show they limit overheating. You usually need the assessment at the design and planning stage, before construction, because the result can change windows, shading and ventilation.
- What is the difference between the Simplified Method and Dynamic Thermal Modelling?
- Approved Document O offers two routes. The Simplified Method limits glazing area and sets minimum opening areas for ventilation; it is quicker but more restrictive. Dynamic Thermal Modelling, a TM59 assessment, simulates the building hour by hour across a design summer and usually gives more design freedom. We advise which route suits your scheme.
- What is a CIBSE TM59 assessment?
- TM59 is the CIBSE standard methodology for assessing overheating risk in homes using dynamic thermal modelling. It tests the building against defined comfort criteria for living spaces and bedrooms, and is the recognised way to demonstrate compliance with Approved Document O by the dynamic route. We model it in-house from your drawings.
- Do you do overheating assessments outside London?
- Yes. An overheating assessment is a calculation and report based on your drawings and location data, so we provide TM59 and TM52 assessments anywhere in the UK without a site visit. We work from your plans, elevations and specification and return the report with any recommended design changes.
- Does Part O apply to my extension or loft conversion?
- Usually not. Approved Document O applies to new residential buildings only, so a straightforward extension or loft conversion to an existing house is outside its scope. It bites when you create a new dwelling: a new build, a conversion, or a change of use that produces homes. If your project creates a separate dwelling, assume it is in scope and check early.
- Which TM59 edition applies, the 2017 or the 2026 one?
- Both matter, for different reasons. Approved Document O lists TM59 [2017] in its documents-referred-to appendix, so that is the edition the 2021 Part O guidance is written around. CIBSE published a new TM59 in July 2026, adding a revised bedroom criterion and testing against 2050s weather data, and recommends it as best practice. We will tell you which your building control body and any planning condition actually expect before we model anything.
- Can I just specify air conditioning instead?
- No, not as a first move. Requirement O1(2)(b) states that mechanical cooling may only be used where insufficient heat is capable of being removed from the indoor environment without it. Part O is a passive-first regime: shading, glazing area, orientation and openable area come first, and cooling is what you justify after those have been shown not to be enough on their own.
- Does the location of my site change the answer?
- Yes, significantly. For the Simplified Method, Part O splits England into a moderate-risk category and a high-risk category covering urban and some suburban parts of London, with further guidance for parts of central Manchester. A high-risk location gets tighter glazing limits, which is why the same house design can pass on one site and fail on another.
- How do the window-opening assumptions work in a TM59 model?
- Part O constrains them so the model cannot be made to flatter the design. For a room occupied during the day, between 8am and 11pm, openings are modelled to start opening above 22°C, be fully open above 26°C, start closing below 26°C and be fully closed below 22°C. Those fixed profiles are a common reason a model that looked fine in-house fails a checked assessment.
- What does building control actually want to see?
- For the dynamic route, a report demonstrating the building passes the TM59 assessment, containing the details set out in TM59 section 2.3. Approved Document O also includes a compliance checklist at Appendix B that designers can use to demonstrate compliance. We produce the report and the checklist together so there is nothing left for you to assemble.