Overheating reporting for a single aspect south facing flat

An overheating assessment for a single aspect south facing flat starts with one decision that determines everything after it: simplified method or dynamic thermal modelling. For this room type the answer is nearly always modelling, because the simplified route treats a single aspect flat as not cross-ventilated, and the glazing and free area limits that follow are very hard to meet on a south elevation.

So the reporting structure below assumes modelling. Set it out in the order a reviewer checks it, which is route, weather, rooms, ventilation, mitigation, results.

Choosing the assessment route

Say which route you used and why, in the first paragraph, before anything else.

Approved Document O provides a simplified method with tabulated limits on glazing area and on the free area available for removing heat, differentiated by location risk and by whether the dwelling is cross-ventilated. A flat with openings on one elevation only is not cross-ventilated, and the tables respond accordingly.

The dynamic modelling route follows the CIBSE guidance for overheating in homes and allows trade-offs. A smaller window with external shading and a lower solar transmittance can compensate for an opening area that the simplified tables would reject. That flexibility is the reason to use it, and it is also the reason the assumptions have to be stated in full.

State the location risk category from the Approved Document as well. The document identifies higher risk locations, and much of London falls within them, which changes both the simplified limits and the expectations a reviewer brings to a modelled result.

Weather files and the scenario tested

Three lines, and reviewers look for all three.

The weather location used, which should be the nearest CIBSE weather site to the development rather than a generic file. The design summer year used, which for compliance purposes is the first design summer year dataset. And the emissions scenario and percentile, which the Approved Document specifies for the current period.

Report them as a table row rather than burying them in a paragraph. Then add one sentence saying whether any additional future scenario was tested. Many authorities now ask for a sensitivity run against a warmer future file, and offering it unprompted is cheaper than being asked for it after committee.

An assessment that names no weather file is not an assessment. It is a set of numbers.

Rooms selected for modelling

Model every occupied room, or state precisely which worst case rooms were selected and on what basis.

For a single aspect south facing flat the critical rooms are obvious: the living room or living kitchen with the largest south glazing, and the bedroom with the highest solar exposure. Include a corner case if the block has one, and include a top floor unit, because roof gains change the answer.

Give each room a reference tied to the drawings, its floor area, its glazed area, its orientation and its openable area. A table with one row per room modelled, and no room left out of it, is the single most useful page in the report.

Where a block contains repeated flat types, say which type each modelled room stands for and how many units it covers. A reviewer needs to know that the whole scheme is covered by the rooms in front of them.

Ventilation assumptions and their honesty

This section is where most reports quietly fail, and where a careful reviewer looks first.

Use the restricted free area of each opening, not the nominal casement size. Restrictors fitted for falling safety, opening limiters on upper floors, and secure night ventilation arrangements all reduce the area available, and a model run on unrestricted openings overstates cooling by a wide margin.

Then deal with noise. Approved Document O sets internal noise limits for bedrooms at night, and where opening a window would exceed them, the modelling must assume the window closed and the design must remove excess heat by another means. That links the overheating work directly to the acoustic work, and the two documents have to share assumptions. Where the site has a noise constraint, the Noise Impact Assessment should state the facade levels the overheating model relied on.

Three further assumptions to declare. Occupancy profile and internal gains, including whether the Approved Document's profiles or bespoke ones were used. Corridor and riser temperatures, which on a communal heating system can be considerably above ambient and which drive gains through the flat entrance door. And whether blinds or curtains were assumed, because assuming occupant behaviour is not a design measure.

Mitigation in order of preference

Report the measures in the order the Approved Document expects them, and show what each contributed.

1. Reduce unwanted solar gains: glazing area and proportion, orientation, solar transmittance, external shading such as brises soleil, reveals, balconies and fins.
2. Remove excess heat: openable area, opening type, stack or single sided ventilation performance, secure night ventilation.
3. Mechanical ventilation, where passive means fall short.
4. Active cooling, only where the earlier steps cannot achieve compliance.

Give a figure for each step. Exceedance hours before and after shading. Exceedance hours before and after increasing openable area. A report that lists measures without showing their effect cannot demonstrate that step four was necessary, and in London the cooling hierarchy in the London Plan requires exactly that demonstration.

There is an honest tension here worth naming. Reducing glazing helps overheating and hurts daylight, so the two assessments pull against each other, and the resolution has to be visible. Where glazing is reduced, check the effect in the Daylight and Sunlight Assessment rather than discovering it later.

Results presentation and pass criteria

Present both criteria, per room, as numbers and as a pass or fail.

The first is the limit on hours of exceedance across living rooms, kitchens and bedrooms during May to September, expressed as a percentage of occupied hours. The second is the night-time criterion for bedrooms, based on operative temperature between 10pm and 7am expressed as a percentage of annual hours.

Do not report a single blended figure. Do not report only the worst room. And do not report a pass without the margin, because a room passing by a tenth of a percentage point is a room that will fail once a restrictor is fitted or a blind is left open.

As an illustrative example, a fourth floor single aspect flat with a 4.2 m² south facing living room window, a 1.1 m balcony overhang, glazing at 0.35 solar transmittance and 1.4 m² of restricted openable area might clear the daytime criterion comfortably and sit close to the night-time bedroom limit. That is the result to state plainly, along with what would push it over.

What the planning submission should state

Keep the planning facing summary to one page, because the case officer is not going to read the model output.

Four items. The compliance route and the criteria applied. The result for every room type, as a table. The design measures the result depends on, listed as commitments. And a sentence confirming that compliance will be demonstrated to building control at the appropriate stage.

That fourth point matters because overheating is a Building Regulations requirement delivered alongside a planning permission, and the planning condition will normally require the measures to be retained. List them explicitly so the condition can be drafted against something: the glazing specification, the shading geometry, the openable areas and any mechanical provision.

Keep the figures consistent with the Energy Statement, because the two models share fabric and ventilation inputs and a mismatch between them is an easy point for an objector to make. And file the full modelling output as an appendix to the Overheating Risk Assessment rather than as a separate document nobody can find.

Enter your postcode on PlanScope to see which reports a residential scheme on your site is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Councils usually expect overheating modelling to be carried out by a suitably qualified author, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.