Part O overheating: simplified method or dynamic modelling
Approved Document O offers two routes. The simplified method is quick but rigid on glazing and free area, and noise or security constraints on opening windows usually push a scheme to dynamic modelling.
Approved Document O gives two ways to show a new home will not overheat: the simplified method, which caps glazing area and sets a minimum free area for openings, and dynamic thermal modelling to CIBSE TM59. The decision between them is usually made for you by the site rather than by preference. If windows cannot be opened at night because of road noise, security at ground level or air quality, the simplified method stops being available in practice and a dynamic model is the only honest route. Making that call at concept stage, rather than at technical design, is what keeps the elevations intact.
Part O has applied to new residential buildings since June 2022, covering dwellings, residential institutions and rooms for residential purposes such as student accommodation and care homes. It does not apply to a material change of use, which is why office-to-residential conversions with deep floorplates and sealed facades remain a live planning issue rather than a Building Regulations one.
What the two compliance routes require
The simplified method works from tables. For each part of a residential building, it sets a maximum glazing area and a minimum free area for openings, both as a proportion of floor area, and it splits the tables by whether the space is cross-ventilated and whether the location is treated as moderate or high risk. High risk covers specified urban parts of London. Meet the figures, provide the shading the tables call for, and you comply.
The modelling route replaces the tables with a simulation. A dynamic thermal model of the building is run against the specified weather data and assessed against TM59 criteria: the hours of exceedance for occupied rooms across the summer period, and a separate night-time criterion for bedrooms. Pass both and the building complies.
Both routes sit under the same overall requirement. Limit unwanted solar gains in summer, and provide an adequate means to remove excess heat from the indoor environment, in that order, with mechanical cooling treated as a last resort rather than a first answer.
Where the simplified method runs out
The tables are strict. On a south or west facing single-aspect flat with generous glazing, the maximum glazing area is often exceeded by the design the planning drawings already show, and the required free area is difficult to achieve with restricted openings.
Four situations push a scheme off the simplified route:
- Glazing area above the table maximum, which is common on single-aspect units designed for a good outlook.
- Free area below the table minimum, usually because openings are restricted to 100 millimetres for fall protection above ground floor.
- Noise, security or air quality that prevents windows being opened at night.
- Any space where cross-ventilation is claimed but the internal layout does not deliver it.
The last one is worth dwelling on. Cross-ventilation in Part O terms means openings on different facades with a usable path between them. A flat with a window on one elevation and a small opening onto an enclosed internal corridor is not cross-ventilated, whatever the plan suggests.
Inputs a dynamic model needs
A TM59 assessment is only as good as its inputs, and the inputs are where reviewers find problems. The model needs the constructed geometry including balconies, reveals and any external shading, the actual glazing specification with g-value and light transmittance, the thermal mass of the build-up, the internal gains profile, the occupancy pattern, and the opening schedule with the free area each opening actually delivers when restricted.
Weather data is prescribed rather than chosen. Approved Document O calls for a design summer year file for the relevant location on a high emissions, fiftieth percentile basis, which matters because using a test reference year instead makes a scheme look considerably cooler than the compliance basis allows. If a submitted model does not state its weather file, that is the first question to ask.
The same discipline applies across the evidence base. Our note on the energy modelling assumptions officers challenge covers the equivalent list for the energy side of the submission.
Noise and security constraints on opening windows
This is the pinch point on most urban sites. Approved Document O expects the overheating strategy to account for night-time noise, and gives indoor guideline levels for bedrooms at night. Where opening a window would breach those levels, the model must treat the window as closed during those hours, which usually removes the ventilation the design was relying on.
The same logic applies to security at ground and lower ground floor, and to sites inside an Air Quality Management Area where opening windows at night is not a sensible mitigation. A scheme can end up with a purge strategy that only works in the daytime and a bedroom criterion that fails at night.
The design answers are limited and they are all architectural. Acoustically attenuated background ventilators with mechanical extract. Secure night vents behind louvres or shutters. Deep window reveals with a protected opening. Or, where nothing else works, mechanical cooling, which then has to be justified against the cooling hierarchy that local plan policy usually imposes.
Illustrative example. A single-aspect one-bedroom flat facing a busy A road, with a 2.4 metre floor-to-ceiling height and full-height glazing, fails the simplified method on glazing area and fails TM59 at night once the bedroom window is modelled as closed. Adding external fixed shading to the glazing, reducing the glazed proportion to around half the facade, specifying a lower g-value and providing an attenuated secure night vent brings it back. The point is not the numbers, which are illustrative, but that all four moves change the elevation.
Glazing, shading and orientation trade-offs
Shading is more effective outside the glass than inside it, and fixed external shading is more reliable than anything the occupant has to operate. Brise-soleil, deep reveals, recessed balconies, projecting floor slabs and fixed vertical fins all count, and they are also elevation design, so they belong in the planning drawings rather than being added later.
Lower g-value glazing helps, but it interacts with daylight and with the Part L calculation, because reducing solar gain in August also reduces useful gain in February. Orientation is the free move: on a mixed-orientation block, putting the deeper plan units on the north and the dual-aspect units on the south and west corners solves more overheating than any specification change.
Thermal mass exposed to the internal air, with night purge to discharge it, remains one of the more reliable passive strategies where the ventilation is actually available. Where the roof is in play, a green roof contributes at roof level, though it does not do much for a fifth-floor south-facing flat.
How overheating evidence reaches the planning file
Part O is a Building Regulations matter, checked by building control, not by the planning officer. That is the formal position, and it is also why schemes get into trouble.
Planning fixes the geometry: glazing proportion, shading depth, balcony form, orientation, window sizes and opening types shown on approved drawings. Change any of those to pass Part O and you may need a non-material amendment under section 96A of the Town and Country Planning Act 1990, or a section 73 application, with the delay that brings.
Many authorities now ask for an Overheating Risk Assessment at application stage, and in London the requirement flows from the cooling hierarchy in the London Plan rather than from the Building Regulations at all. Where that is the case, the overheating evidence sits alongside the Energy Statement and the Sustainability Statement in the submission, and they should tell the same story about glazing and ventilation.
Conditions and verification
Expect conditions that fix the things the assessment relied on: the glazing specification including g-value, the external shading as drawn, the opening types and restrictor arrangements, and any mechanical ventilation or cooling plant with its noise limits.
Two practical habits help at discharge stage. Keep a schedule mapping each assumption in the model to the drawing or specification clause that delivers it, so the discharge submission writes itself. And flag early any assumption that the contractor is likely to value engineer, because a swapped glazing unit or a deleted brise-soleil invalidates the compliance case and nobody notices until handover.
PlanScope can draft the planning documents around this evidence, including the overheating and energy statements, with missing site facts flagged rather than invented. Thermal modelling should be carried out by a suitably qualified assessor, and PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice: check the position with your local planning authority and your building control body.
Frequently asked questions
Does Part O apply to an office to residential conversion?
Not directly. Part O applies to new residential buildings, and a material change of use falls outside its scope, which is why deep-plan conversions with sealed facades can still produce homes that overheat. Local plan policy and the planning officer may still ask for an overheating assessment, and in London the cooling hierarchy in the London Plan commonly drives that request.
Can I use a test reference year weather file for TM59?
Not for Part O compliance. Approved Document O calls for a design summer year file for the relevant location on a high emissions, fiftieth percentile basis. A test reference year describes an average year rather than a warm one, so it produces a considerably more favourable result. Any model that does not state which weather file it used should be queried.
What counts as cross-ventilation under Part O?
Openings on different facades with a usable air path between them, so that air can pass through the space. A window on one elevation and an opening onto an enclosed internal corridor does not qualify. Getting this wrong matters because the simplified method sets different glazing and free area figures for cross-ventilated and non-cross-ventilated spaces.
Is mechanical cooling acceptable?
It is permitted, but it sits last in the hierarchy. Part O expects solar gains to be limited first and excess heat to be removed by passive means where possible. Local plan policies, particularly in London, apply a cooling hierarchy that requires the earlier steps to be shown as exhausted before active cooling is accepted, with the energy and carbon consequences set out.
Do planning drawings need to change if Part O fails at technical design?
Often, yes. The fixes are usually less glazing, deeper external shading, different opening types or a changed balcony form, all of which appear on approved elevations. Depending on the extent, that means a non-material amendment under section 96A or a section 73 application. Testing the strategy before the planning drawings are frozen avoids the delay.
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.