Part O overheating: what the 2021 amendment introduced

The Part O overheating requirement applies to new residential buildings in England and it is a building regulations matter, yet it changes drawings that were fixed at planning. Approved Document O supports requirement O1 of Schedule 1 to the Building Regulations 2010, introduced by the 2021 amendment regulations, and it asks for two things: limit unwanted solar gains in summer, and provide an adequate means of removing excess heat.

Neither of those is free on a south facing single aspect flat. That is why the requirement has to be tested at concept stage rather than after the permission is granted.

Why an overheating requirement was added

Dwellings had become better insulated and more airtight without a corresponding rule about summer conditions. A building that holds heat in January also holds it in July, and the energy standards said nothing about the second case.

The gap showed up most in single aspect flats with large windows and restricted openings, particularly where noise or security meant those windows stayed shut. Part O closed the gap by setting a requirement in the regulations rather than leaving it to guidance.

Which buildings Approved Document O applies to

New residential buildings. That covers dwellings, residential institutions, and buildings containing one or more rooms for residential purposes such as student accommodation and some care settings.

It does not apply to work in existing buildings, and it does not apply on a material change of use. A converted office becoming flats under the prior approval route is outside Part O, which is an odd outcome given that such conversions produce exactly the deep plan single aspect units the requirement was written for. Planning policy is where that gap gets addressed, and many authorities now ask for an overheating assessment on conversions for that reason.

The simplified method and where it runs out

The simplified method is a set of tabulated limits. It caps the glazing area as a proportion of floor area, with the cap varying by orientation and by whether the dwelling is cross ventilated or single aspect, and it sets a minimum free area of openings for removing heat. Approved Document O distinguishes moderate risk and high risk locations, with the high risk category applying to specified areas, chiefly in London.

It is quick, it needs no software and no consultant, and it suits a conventional house type with modest glazing. It runs out quickly on anything else. A living room with a full height glazed wall onto a terrace will not pass. Nor will a single aspect flat whose only openings face a road where the acoustic strategy requires windows to be closed at night.

When the simplified method fails, the honest reading is that the design has a summer problem, and the modelling route is a way of demonstrating the solution rather than a way of avoiding the issue.

The dynamic modelling route

The alternative is dynamic thermal modelling to the CIBSE methodology, using TM59 for domestic buildings, with design summer year weather files for the relevant location and the specified internal gain profiles. The assessment reports whether the criteria are met in occupied rooms, with bedrooms tested separately for night time conditions.

Three inputs decide most outcomes. The weather file and the location it represents. The assumed openable area and whether windows can actually be opened at night given noise, security and fall protection. And the internal gains, which are set by the methodology rather than chosen.

Modelling is not a rescue for a scheme that has already been consented with the wrong glazing. It is most useful early, run on two or three options, to tell the design team how much glass the plan can carry. Our note on reporting TM59 results in an overheating risk assessment sets out what a readable output looks like.

Where planning and building control overlap

Part O is enforced through building control. The planning authority does not apply it. But almost every mitigation Part O points to is something the planning permission fixes.

External shading, brise soleil, deep window reveals, balcony depth and position, window size and proportion, the orientation of habitable rooms, the position of mechanical plant, and the ability to open a window at night where there is a noise constraint. All of those appear on approved drawings. Changing them afterwards means a section 73 application or, at best, an awkward conversation about whether the change is non-material.

London schemes have carried an overheating requirement through the London Plan cooling hierarchy for some years, and authorities elsewhere increasingly ask for an assessment at application stage. Where they do, the Overheating Risk Assessment is the document, and it should use the same methodology and the same assumptions that building control will later see.

Illustrative example: a block of single aspect flats facing a busy road achieved its acoustic standard with a closed window strategy and mechanical ventilation. The planning drawings showed full height glazing to the living rooms. The Part O assessment then required either a reduction in glazed area or active cooling, and the external units for cooling had nowhere to go on an approved elevation. The scheme went back for a variation, and the delay ran to months.

What it does to glazing, shading and balconies

Four moves do most of the work, and all four are architectural rather than mechanical.

1. Reduce glazing on the south and west elevations, or break it up so the g-value can be tuned without making the whole facade dark.
2. Shade externally. A fixed horizontal projection over a south facing window works and needs no maintenance; internal blinds do almost nothing for solar gain.
3. Use balconies as shading by giving them depth and placing them above the window they protect rather than beside it.
4. Design the openings for night purge, with secure and safe openable area that the acoustic and fall protection strategies actually permit.

Specifying glass with a very low g-value across the whole facade is the fallback, and it is a poor one because it darkens the interior in winter and reduces useful solar gain. Treat it as the last option rather than the first.

Evidence to submit at each stage

At planning, where the authority asks for it, submit the assessment with the methodology named, the weather file identified, the openable areas stated and cross referenced to the window schedule, and the acoustic and fall protection constraints set out explicitly. Where the site is constrained, the approach in overheating assessments on a constrained urban site is the relevant one.

At building control, submit the chosen compliance route with its supporting calculation, plus the information Approved Document O asks to be provided to the owner about how the overheating strategy is meant to be operated. Check the current edition of the Approved Document before relying on any tabulated figure, because the guidance has been amended since it was first issued.

Councils usually expect a suitably qualified author for a dynamic thermal model, and professional review of any draft assessment is strongly advised.

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