Overheating assessments on a constrained urban site
TM59 and Approved Document O assume a free running strategy that constrained urban sites cannot deliver. The design moves that pass, and the ones that only look like they do.
Urban overheating assessments fail for a predictable reason: the modelling assumes windows open at night and the site does not allow it. Put a block of single aspect flats beside a bus route inside an Air Quality Management Area, and the free running strategy that would pass on a suburban site collapses. Approved Document O and CIBSE TM59 both anticipate this, and both push the design towards fixing the problem in the layout rather than in the plant.
Get the constrained variables on the table at concept stage. Once the massing is fixed, the remaining moves are expensive and marginal.
What the assessment is actually testing
TM59 sets two criteria for predominantly naturally ventilated homes. Criterion A applies the adaptive comfort approach from TM52 to occupied rooms across May to September, allowing exceedance for no more than 3 per cent of occupied hours. Criterion B looks only at bedrooms between 22:00 and 07:00 and allows no more than 1 per cent of annual night hours above 26 degrees. For homes that rely on mechanical ventilation, TM59 substitutes fixed temperature criteria across occupied rooms.
Approved Document O offers two routes: a simplified method with maximum glazing areas and minimum free ventilation areas depending on whether the location is high risk, and dynamic thermal modelling following TM59. Which one applies, and where the two diverge, is set out in the comparison of TM59 against Approved Document O.
The planning question sits earlier than either. A council asking for an Overheating Risk Assessment at application stage wants to know that the approved massing can meet the criteria without a redesign at Building Regulations stage.
Single aspect and deep plan layouts
Single aspect flats cannot cross ventilate. That is the whole of the problem in one sentence. A south or west facing single aspect unit with a high glazing ratio and no external shading will fail Criterion A on almost any urban site, and dual aspect corner units will mask the failure if you only model a sample.
Model the worst unit, not the average one. The units that matter are top floor, west or south facing, single aspect, with the largest glazing ratio and the smallest openable area.
Deep plans compound it. A living space more than about 6 metres deep from the only opening moves very little air by stack or wind effect, so the nominal free area on the drawing does not translate into measured ventilation. Where the plan has to be deep, the answer is usually a dual aspect arrangement or a ventilation path through a lobby, both of which are planning decisions about circulation cores rather than services decisions.
When noise and air quality close the windows
Approved Document O treats bedrooms as likely to have windows closed at night where internal noise would exceed 40 dB LAeq,8hr, or 55 dB LAFmax more than ten times a night. If your acoustic consultant reaches that conclusion, the overheating strategy cannot rely on openable windows at night, and the modelling has to be run on that basis.
This is where the Noise Impact Assessment and the overheating work have to be done by people who are talking to each other. A scheme that submits an acoustic report recommending closed windows with acoustic ventilators, and an overheating report assuming windows open to 45 degrees at 2am, has contradicted itself in public.
Air quality has the same effect in a different way. Where a facade sits close to a busy road inside an AQMA, the environmental health officer may resist reliance on opening windows for cooling at all, and the scheme is pushed towards filtered mechanical supply.
Passive moves, ranked by what they actually deliver
The London Plan cooling hierarchy is a good ordering even outside London: reduce the heat entering the building, minimise internal heat generation, manage heat within the building, use passive ventilation, then mechanical ventilation, and only then active cooling.
In practice, on a constrained site:
- Orientation and unit mix. Put single aspect units on the north side and dual aspect on the corners and the hot elevations.
- External shading. Balconies as shading devices, deep reveals, brise soleil and fixed horizontal fins on south elevations, vertical fins on west.
- Glazing ratio. Reducing the window to wall ratio on the hot elevations does more than upgrading the glass, and costs less.
- Solar control glass, chosen on g-value with an eye on light transmittance, because a low g-value that darkens the room brings artificial lighting back as an internal gain.
- Exposed thermal mass with secure night ventilation, which only works if the openings can stay open safely.
- Insulating and controlling heat network pipework, which is often the largest internal gain in a flat.
Green roofs and high albedo surfaces help the microclimate and contribute to the Energy Statement narrative, though they rarely move the TM59 result on their own.
Communal corridors, the failure nobody models early
Corridors served by heat network risers are frequently the hottest part of a residential block. TM59 asks for communal corridors to be assessed, with a working expectation that they do not exceed 28 degrees for more than 3 per cent of annual hours, and on many schemes the first model run comes back far outside that.
The fixes are pipework insulation to a higher standard than the default, moving risers out of the corridor, mechanically ventilating the corridor with a proper extract path, and reducing the number of heat interface units opening onto it. Each of those has a planning consequence for core layout and facade openings, which is why it belongs in the design stage rather than in a condition. Where a connection to a network is being required, the drivers are explained in the note on heat network zoning.
An illustrative example
Illustrative only: a nine storey block of 48 flats on a corner site, with a main road on the south elevation and a railway to the west. The first model showed 21 of 48 units failing Criterion A, all single aspect and south facing. The design team reduced the south glazing ratio, extended balconies to give 900mm of horizontal shading, swapped six single aspect units to dual aspect by moving the core, and specified acoustic ventilators with a mechanical extract for the closed window condition.
Active cooling was avoided. The changes were drawn before the planning submission, so the overheating report supported the approved drawings rather than qualifying them.
Mechanical ventilation with cooling as the fallback
Sometimes the site wins. A deep urban plot with noise on three sides and no scope to change the massing may need mechanical ventilation with a cooling coil in the worst units. Say so openly in the assessment, show the hierarchy you worked through first, and quantify the added regulated energy so the energy statement stays consistent.
What does not work is presenting active cooling as a design choice without evidence of the passive steps tested and rejected. Planning officers read that as a shortcut, and in London it conflicts directly with the cooling hierarchy.
Run the model at concept stage on the three worst units. It costs a day and it saves a redesign.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Why do single aspect flats fail overheating assessments?
They cannot cross ventilate, so the only route for purging heat is one facade. On a south or west elevation with a high glazing ratio and no external shading, solar gain accumulates faster than a single opening can remove it. Modelling a sample that includes dual aspect corner units hides the problem, so test the worst unit instead.
What happens if noise means windows have to stay shut at night?
The overheating model has to be run on the closed window basis. Approved Document O treats bedrooms as likely to be closed where internal noise would exceed 40 dB LAeq,8hr or 55 dB LAFmax more than ten times a night. That usually pushes the scheme towards acoustic ventilators, mechanical supply or, in the worst units, a cooling coil.
Does a green roof help the overheating result?
Marginally for the dwellings below, and more for the local microclimate and the surface water strategy. It rarely changes a TM59 outcome on its own, because the dominant gains in a flat are solar through the glazing and heat from network pipework. Treat it as part of the sustainability case rather than as the overheating fix.
Are communal corridors assessed under TM59?
Yes. TM59 asks for communal corridors to be assessed, with a working expectation of no more than 3 per cent of annual hours above 28 degrees. Corridors carrying heat network risers often fail on the first run, and the remedies involve pipework insulation, riser position, corridor ventilation and core layout rather than anything that can be added later.
When is active cooling accepted at planning stage?
Where the assessment shows the passive steps in the cooling hierarchy have been worked through and the site still cannot meet the criteria, usually because noise, air quality or plan depth prevent night purging. Set out what was tested and rejected, and quantify the added regulated energy so the energy statement remains consistent with the overheating report.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.