Passive cooling features and how officers assess them

Officers assess passive cooling the way they assess anything else: by looking at the drawings. That is the practical point behind every passive cooling planning assessment. A shading strategy that exists only in the thermal model, with no brise soleil on the elevation and no reveal depth on the section, has no planning consequence and cannot be conditioned. A strategy drawn accurately changes the elevation, which means the design and the environmental case have to be resolved together rather than in sequence.

Approved Document O requires new residential buildings to limit solar gains and to provide means of removing excess heat, with passive means preferred over mechanical cooling. The London Plan sets out a cooling hierarchy that starts in the same place. Both push the work upstream into the design, which is where planning sees it.

Shading devices and the elevation

Fixed external shading is the measure that does most for overheating and most to the elevation.

Horizontal shading works on south facing glazing, because the summer sun is high. Depth is a function of window height and the solar altitude you are designing for, so a 300 mm overhang on a 2.4 metre tall window does very little. East and west facing glazing is the harder problem: low sun angles mean horizontal shading is close to useless and vertical fins or external shutters are needed.

What officers look at:

• Whether the shading is shown on every elevation, at the right depth, with a section confirming it.
• Whether it reads as part of the architecture or as equipment bolted on afterwards.
• Whether it is fixed or operable. Fixed shading is permanent and enforceable. User-operated blinds and shutters depend on behaviour and are given less weight.
• Whether balconies and recesses have been counted as shading, and whether the model reflects their actual geometry.
• In a conservation area or on a listed building, whether external shading is acceptable at all, which it frequently is not.

That last point is the real constraint on retrofit and on infill in historic areas. Where external shading is ruled out, the strategy has to lean on glazing specification, orientation and ventilation instead, in the same way that fabric upgrades have to work around appearance, as discussed in our note on external wall insulation in a conservation area.

Thermal mass and exposed soffits

Thermal mass absorbs heat during the day and releases it overnight, which flattens the peak temperature. It only works if two conditions hold: the mass has to be exposed to the room air, and the building has to be purged at night so the mass can discharge.

Exposed concrete soffits are the usual route, and they collide with three other requirements. Services distribution wants a ceiling void. Acoustic separation between flats wants a resilient layer. And residential buyers often want a flat white ceiling rather than fair-faced concrete.

The workable compromises are partial exposure, with a perimeter margin for services, or exposed soffits in living spaces only. Say which you are doing and show it on the section, because a model that assumes full exposure against a drawing showing a suspended ceiling throughout will not survive a careful reading of the Overheating Risk Assessment.

Lightweight construction can still meet the criteria. It simply has to do so through shading, glazing ratio and ventilation rather than through mass, and the modelling has to say so.

Night purge ventilation and security

Night purge is where most residential strategies are tested, because the ventilation opening that discharges the building at 3am is the same opening a resident will not leave open on a ground floor flat beside a footpath.

Approved Document O expects means of removing excess heat to be usable while maintaining security, and it treats external noise as a constraint on opening windows. So the design has to provide purge ventilation that is secure and acceptable acoustically. The options:

1. Restricted opening lights at high level, above reach from outside, with a free area calculated rather than assumed.
2. Secure louvred vents behind a perforated screen, which is a visible elevational element and needs drawing.
3. Acoustically attenuated trickle and purge vents where road or rail noise prevents open windows.
4. Dual aspect layouts with cross ventilation, which is a plan decision made at the earliest stage.
5. Mechanical ventilation with a summer bypass, which is not passive and should be presented honestly as the fallback it is.

Single aspect flats facing a busy road, deep in plan, with no external shading permitted, are the combination that fails. If that is the layout, the honest answer is active cooling, and it is better to say so in the energy statement than to model an opening nobody will use.

Orientation and glazing ratio

The lowest cost passive cooling measure is a plan that faces the right way with a sensible amount of glass, and it costs nothing if the decision is made early enough.

Officers will sometimes ask for glazing percentages by elevation. It is a fair question and worth putting in the Energy Statement before it is asked, alongside the g-value assumed in the model.

An honest note on glazing. Large west facing windows sell flats and cause overheating, and the tension between the two is rarely resolved by the environmental consultant. It is resolved by whoever decides the sales strategy.

Landscape and the wider site

Site-scale measures have a smaller effect on a single dwelling and a real effect on the outdoor environment and on urban heat.

Tree canopy over amenity space and car parking reduces surface temperatures and is straightforward to condition through a landscape scheme. Lighter coloured paving and roofing reduces absorbed heat. Green roofs moderate surface temperature and help surface water attenuation at the same time, and combining them with photovoltaic panels is discussed in our note on biosolar roofs.

Water features are usually the wrong answer on a residential scheme: maintenance liability, legionella management and winter appearance outweigh a marginal cooling benefit. Shade and planting do more for less.

Where a greening factor or urban greening policy applies, the landscape strategy and the overheating strategy should be prepared together, because the same tree can serve both.

Evidencing the contribution

Modelling is where the passive measures become numbers. Two rules keep an assessment credible.

Model what is drawn. Every shading element, reveal depth, opening area and exposed surface in the model should correspond to something on the drawings, and where it does not the drawings should change.

State the assumptions that the reader cannot see: occupancy pattern, internal gains, window opening behaviour, whether curtains or blinds were assumed, and the weather file used. Officers rarely interrogate these, and the consultant reviewing on the council's behalf will.

Then condition it. A condition requiring the development to be built in accordance with the approved shading and ventilation details, with any change to be agreed in writing, is what stops the brise soleil being value engineered out in month eight.

Where passive measures fall short

Three situations, reliably:

Deep plan single aspect dwellings with one orientation and no cross ventilation. Sites where noise or air quality requires windows to stay shut. And retrofit in historic fabric where external shading is unacceptable and glazing cannot be changed.

In each of those, the right move is to say so, show the passive measures taken as far as they go, and present the mechanical element as the residual response rather than the starting point. Officers respond far better to that than to an optimistic model, and the alternative is a condition requiring a revised strategy after permission, which is worse for everybody.

If you are preparing the energy and overheating documents for a scheme, enter your postcode on PlanScope to see which reports the site is likely to need.

This is general guidance, not legal advice. Overheating modelling is specialist work and councils usually expect a suitably qualified author, so 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.