Overheating Assessments:
Covering England and Wales

An overheating assessment is a report that evaluates whether a building is at risk of becoming too hot in summer and what mitigation is needed to reduce that risk. In building-regulations terms, it is the process used to show compliance with Part O for qualifying residential buildings. In planning terms, it is often a dynamic thermal modelling exercise used to show how the scheme manages heat risk through design rather than relying on late mechanical cooling.

Not always. A Part O assessment is specifically the Building Regulations route for qualifying new residential buildings. An overheating assessment can also be a planning-stage exercise, especially in London, where dynamic modelling is required on major developments even if the design team later uses a different compliance route for Building Regulations.

Yes, in current England guidance it does. Approved Document O says the guidance applies to new residential buildings only. The same core position exists in Wales, where the current Approved Document O says it applies when a new residential building is erected. That makes Part O a new-build residential overheating regime, not a blanket rule for all building work.

No, not currently in England. The official England FAQ says Part O does not apply to buildings undergoing a change of use, and the 2023 consultation documents also state that MCU dwellings are currently outside scope. In Wales, the current Approved Document O applies when a new residential building is erected, so the live Welsh scope is also not a blanket change-of-use route.

Yes. The GLA Energy Assessment Guidance says London Plan Policy SI 4 requires all major development proposals to undertake dynamic overheating modelling. It also says this dynamic modelling is required at the planning application stage, regardless of which method may later be used to demonstrate compliance with Approved Document O.

The simplified method is the prescriptive Part O route based on glazing, location, shading and free-area limits. Dynamic thermal modelling is the performance route using a simulated building model to predict overheating risk. England’s Approved Document O says compliance can be demonstrated by either the simplified method in Section 1 or the dynamic thermal modelling method in Section 2. Wales also offers the same two broad routes.

TM59 is CIBSE’s Design methodology for the assessment of overheating risk in homes. CIBSE’s Knowledge Portal still lists TM59 (2017) as active, and England’s Approved Document O says the dynamic thermal modelling route should follow CIBSE’s TM59 methodology. In practical terms, TM59 is the standard residential dynamic overheating method most teams mean when they ask for an overheating assessment.

The cooling hierarchy is the policy sequence used to reduce heat risk before falling back on active cooling. The GLA guidance sets it out as: reduce heat entering the building, minimise internal heat generation, manage heat within the building, provide passive ventilation, provide mechanical ventilation, and only then provide active cooling systems. In practical terms, it is a design-first approach, not an air-conditioning-first approach.

Yes, but only as a last resort. Approved Document O says the building should meet Requirement O1 using passive means as far as reasonably practicable, and that mechanical cooling is expected to be used only where the requirement cannot be met using openings. The GLA cooling hierarchy makes the same point by putting active cooling at the end of the sequence.

Treat overheating as an early design issue, not a late compliance add-on. The strongest approach is to review heat risk before the façade and services strategy are fixed, follow the cooling hierarchy, model the right sample units, and keep the analysis live as the design evolves. The GLA and CIBSE guidance both point the same way: early passive design decisions are what stop overheating becoming a late-stage problem.