Overheating assessment for a planning application

An overheating assessment submitted with a planning application is usually there because the mitigation changes the elevations. Part O of the Building Regulations controls overheating in new residential buildings and is a building control matter, but shading devices, reduced glazing ratios, dual aspect layouts and ventilation openings all show on the drawings, so authorities with a heat risk policy want to see the assessment before permission is granted rather than afterwards. Submitting a scheme whose overheating strategy has not been tested is how a planning permission ends up needing amended elevations at Part O stage.

The London Plan sets out a cooling hierarchy and asks for dynamic modelling on major schemes. Outside London the requirement varies by authority, so check the local plan and the validation list before deciding whether to spend the fee.

When an authority asks for an overheating assessment

Three triggers cover most cases. A local or regional policy on managing heat risk that requires an assessment for residential development above a stated scale. A site where noise or air quality mitigation restricts openable windows, since that removes the simplest cooling route. And a scheme design that clearly raises the risk, meaning high glazing ratios, single-aspect south or west facing flats, or deep plan layouts with limited cross ventilation.

A fourth trigger is a pre-application comment. Where an officer has raised heat risk, answering it with modelling is far more productive than answering with assertions about the specification.

Vulnerable occupancy changes the calculus too. Sheltered housing, care accommodation and student schemes carry different occupancy patterns and different tolerance, and an Overheating Risk Assessment for those uses should say so explicitly rather than modelling a generic residential profile.

Dynamic modelling and the simplified method

Part O offers two routes and they are not equivalent in what they tell you.

The simplified method sets limits on glazing area and requires minimum free areas for ventilation, assessed by room type and orientation. It is quick, and on a compliant scheme it is enough for Building Regulations. What it will not do is tell a planning officer how the building performs, because it is a prescriptive check rather than a performance prediction.

Dynamic thermal modelling to the CIBSE methodology simulates the building hour by hour across a summer, and reports whether each assessed room meets the criteria. For planning purposes this is what carries weight, because it produces results per room that can be tabulated and interrogated.

On anything with an unusual plan, a restricted ventilation strategy or an officer asking questions, go straight to modelling.

Weather files and future climate scenarios

The weather file chosen drives the result, and a report that does not name it is not a report you can rely on.

Modelling uses design summer year files, which represent warm summers rather than average ones, and the choice between the available years and the choice of location file both matter. London files are warmer than northern ones, and an urban heat island weighting applies in dense city locations.

Future climate is where authorities increasingly push. Modelling against a projected future scenario for a mid-century or later period is a more demanding test, and some policies require it for the assessment period of the building rather than for today's climate. State which scenario, which period and which emissions pathway you used, because two reports using different assumptions are not comparable and an officer comparing them will ask.

Where noise and air quality close the windows

This is the tension that defeats otherwise good schemes, and it deserves to be confronted rather than managed around.

A flat facing a busy road needs closed windows to meet internal noise criteria, and closed windows remove purge ventilation, which is the primary passive cooling strategy. The same logic applies where air quality at the facade requires filtered mechanical ventilation. Where that applies, the modelling has to assess the closed-window case, and passing with windows open while failing with them closed is not a pass.

Getting the two assessments to talk to each other is the practical answer. Commission the acoustic work and the overheating work at the same time, from consultants who will exchange assumptions, and make sure the facade specification in the Noise Impact Assessment matches the one in the thermal model. A scheme that solves noise with sealed facades and solves overheating with openable windows has solved neither.

Design responses ranked by what they achieve

Work down the hierarchy rather than reaching for cooling.

1. Reduce solar gain through orientation and plan form, which costs nothing at sketch stage and everything later.
2. Reduce the glazing ratio on south and west elevations, and specify glass with a low solar factor where glazing must stay large.
3. Add external shading, since brise soleil, deep reveals, balconies and fixed fins all work far better than internal blinds.
4. Reduce internal gains by insulating and ventilating heating and hot water distribution, which on communal systems is often the single largest contributor.
5. Provide cross ventilation through dual aspect layouts and, where a facade must stay closed, a ventilation route that does not rely on it.
6. Expose thermal mass where the construction allows it and night purge is available.
7. Only then consider active cooling, and expect to justify it.

Point four is the one architects underestimate. Poorly insulated risers and uninsulated heat interface units in flats can push corridor and hallway temperatures well above anything the glazing is doing, and no amount of shading fixes it.

Presenting results a planner can read

An officer is not going to open a simulation file. Give them a document that stands alone.

Tabulate the assessed rooms with their orientation, aspect and result, and show the failures rather than only reporting an overall pass. State the weather file, scenario and period, the ventilation assumption for each room, and the specification the model used. Include the mitigation schedule as a list of physical items that appear on the drawings, so the officer can check one against the other. And add a short paragraph saying what happens if a specification item changes, because that is the question a condition will be written to answer.

Where the scheme also carries an energy strategy, keep the two documents consistent. A Sustainability Statement claiming passive design while the thermal model relies on mechanical cooling is a contradiction an officer will spot.

Conditions that secure the mitigation

Authorities secure overheating mitigation through conditions on the physical items, and that is the right approach.

Expect conditions on glazing specification including solar factor, on external shading devices with dimensions, on the ventilation strategy, and sometimes on a post-completion verification. Ask for wording that allows equivalent or better performance rather than naming a product, and ask for the shading to be described dimensionally so a substitution that changes the geometry is caught.

Where the scheme is all electric with no gas connection, say in the assessment how the heating system contributes to internal gains in summer, since heat pump systems with well-insulated distribution behave differently from communal heat networks. And if photovoltaic panels and shading are competing for the same roof or facade area, resolve that before submission, because combining panels with a green roof shows how quickly roof space runs out.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and dynamic thermal modelling should be carried out by a suitably qualified assessor.