Overheating: Approved Document O against CIBSE TM59
Approved Document O and TM59 answer different questions with different criteria. A scheme can pass one and fail the other, and officers increasingly ask for both.
Approved Document O is a Building Regulations requirement with two compliance routes: a simplified method based on glazing areas and free opening areas, and a dynamic thermal modelling route. CIBSE TM59 is a design methodology for assessing overheating risk in homes, using dynamic simulation against adaptive comfort criteria from CIBSE TM52 plus a bedroom night-time criterion. The modelling route in Approved Document O is derived from TM59 but is not the same assessment, and the differences in weather file, criteria and assumptions mean a dwelling can satisfy one and fail the other.
Planning officers in a growing number of authorities ask for TM59 results at application stage. Building control asks for Approved Document O compliance. Both can be needed.
What Approved Document O requires and when
Approved Document O applies to new residential buildings, including dwellings, institutions and rooms for residential purposes, and to buildings created by a material change of use to residential. It does not apply to extensions to existing dwellings.
The requirement has two limbs: limit unwanted solar gains in summer, and provide an adequate means of removing heat from the indoor environment. There is also a requirement to deal with noise and security where the strategy relies on opening windows, and to provide information to the occupier.
The two routes to demonstrating compliance are the simplified method and dynamic thermal modelling. Either is acceptable to building control.
The simplified method and its limits
The simplified method sets maximum glazing areas as a proportion of floor area, and minimum free areas for openings, varying by whether the location is high or moderate risk and whether the dwelling is cross-ventilated.
It is quick, needs no software, and it is conservative. Schemes that pass it are usually comfortable. Schemes that fail it may still be fine, which is the point at which modelling becomes worthwhile.
Where it struggles is single-aspect dwellings, which many apartment schemes contain in quantity. A single-aspect flat cannot be cross-ventilated, so the glazing limits tighten sharply, and in high risk locations the permitted glazing area becomes small enough to conflict with daylight requirements.
That conflict is real and unresolved. A deep single-aspect flat may need more glass for daylight under BS EN 17037 than Approved Document O will allow without modelling. Modelling is the way out, not a reason to ignore one of the two.
Dynamic thermal modelling under TM59
TM59 assesses predominantly naturally ventilated homes against two criteria. The first, from TM52, is that the number of hours during which the operative temperature exceeds the comfort threshold by one degree or more must not exceed 3 per cent of occupied hours from May to September. The second applies to bedrooms: the operative temperature between 10pm and 7am must not exceed 26 degrees for more than 1 per cent of annual hours in that period.
Mechanically ventilated homes are assessed against a fixed temperature criterion instead.
The methodology prescribes occupancy profiles, internal gains, window opening behaviour and the treatment of blinds, which is what makes results comparable between consultants. It also sets out how to handle communal corridors with heat network losses, which is a frequent failure point in apartment buildings and one that Approved Document O does not address at all.
Corridor temperatures matter more than people expect. A poorly insulated heat network riser in an unventilated corridor raises the temperature of the adjoining flats through their entrance doors and walls.
Where the two approaches disagree
| Point of difference | Approved Document O | CIBSE TM59 |
|---|---|---|
| Status | Building Regulations requirement | Design methodology |
| Weather data | Prescribed design summer years for listed locations | Design summer year chosen for the location, often with future weather runs |
| Criteria | Fixed hours above threshold temperatures | TM52 adaptive criterion plus a bedroom night criterion |
| Scope | Dwelling level | Can include corridors and communal areas |
| Window opening | Prescribed assumptions including noise and security limits | Prescribed profiles, with openings restricted where justified |
The most common source of divergence is the window opening assumption. Approved Document O requires that where windows cannot be opened because of noise, pollution or security, the strategy must work without relying on them. A TM59 run that assumes windows fully open on a street with high traffic noise gives an optimistic answer that neither building control nor a cautious planning officer should accept.
The second common divergence is future weather. Planning policy in some authorities asks for a future weather scenario run, which Approved Document O does not require. A scheme that passes on current weather can fail on a 2050 scenario, and that is precisely why the request is made.
Noise, security and the opening area assumption
This is the detail that decides most marginal cases. If the acoustic assessment concludes that windows must remain closed to meet internal noise guideline values, the overheating model cannot assume them open.
The resolution is usually acoustic ventilation: attenuated trickle or background ventilators, acoustically treated openings, or mechanical ventilation with a purge capability. Each has a cost and a space requirement, and each has to appear on the planning drawings if the officer is to rely on it.
Security constrains ground floor and accessible windows. A window restricted to 100mm opening provides a fraction of the free area of an unrestricted one, and the model must reflect the restrictor.
Run the acoustic and overheating assessments together with the same window assumptions. Two reports that assume different things about the same window is the single most common inconsistency in a submitted pack, and reviewers look for it.
What a planning officer needs as against building control
Building control wants compliance with Approved Document O by one of the two routes, demonstrated at the appropriate stage.
A planning officer wants to know that the design will be comfortable without retrofitted air conditioning, that the mitigation is visible in the drawings and can be conditioned, and that the strategy is consistent with the acoustic and energy strategies. In London, policy asks for the cooling hierarchy to be followed, with passive measures before active cooling.
Give the officer a short summary: the criteria used, the weather file, the proportion of rooms passing and failing, the specific units that fail and why, and the measures incorporated. Then put the modelling output in an appendix. An Overheating Risk Assessment structured that way is usable, and councils normally expect a suitably qualified author.
The Energy Statement should cross-refer rather than repeat, and where a Sustainability Statement is also required, keep the overheating content in one place.
Design responses that survive value engineering
The measures that get cut late are usually the ones that were never drawn properly. Protect them by making them architectural rather than specification items.
- External shading formed by the building, such as recessed balconies, deep reveals, overhangs and fins, which cannot be value engineered without a planning amendment.
- Dual aspect layouts wherever the plan allows, since cross ventilation is the cheapest cooling available.
- Solar control glazing with the g-value stated on the window schedule and in the condition.
- Openable areas dimensioned on the elevations, including restrictor positions.
- Exposed thermal mass where the construction allows, with the finish specified so it is not later covered.
- Insulated and ventilated communal corridors with heat network losses modelled.
Ask for a condition requiring the approved overheating strategy to be implemented. It gives the design team something to point at when the contractor proposes an alternative window.
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This is general guidance rather than 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. For specialist technical reports, qualified professional input may be required.
Frequently asked questions
Does Approved Document O apply to extensions?
No. It applies to new residential buildings, including dwellings, institutions and rooms for residential purposes, and to buildings created by a material change of use to residential. Extensions to existing dwellings fall outside it, although the underlying overheating risk from a heavily glazed extension is unchanged and worth checking.
Is TM59 a legal requirement?
No. TM59 is a CIBSE design methodology rather than a regulation. The modelling route in Approved Document O is derived from it but uses its own criteria and prescribed weather data. Many local plan policies and some London requirements ask for TM59 results at planning stage, which is where the obligation usually comes from.
Why can a scheme pass one and fail the other?
Different weather files, different criteria and different assumptions. Approved Document O prescribes design summer years for listed locations and sets fixed thresholds. TM59 uses the TM52 adaptive criterion plus a bedroom night criterion and can be run on future weather scenarios. Window opening assumptions are the most frequent cause of divergence.
What if the acoustic report says windows must stay closed?
Then the overheating model cannot assume they are open. Approved Document O is explicit that the strategy must work without relying on openings where noise, pollution or security prevent them. The usual answer is acoustically attenuated ventilation or mechanical purge, which needs space and cost allowed for in the design.
What should the planning submission contain?
A short summary stating the criteria, weather file, pass and fail rates by room type, the units that fail and the mitigation incorporated, with the full modelling output in an appendix. Make the mitigation visible on the drawings so it can be conditioned, and keep the window assumptions consistent with the acoustic 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.