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Part O overheating: a plain-English guide to the Simplified Method
Building regulations changed in 2022. Every new home built in England now has to pass an overheating check — and for most projects, that starts with the Simplified Method under Part O. This post explains what the Simplified Method actually assesses, what the numbers mean for your project, and when you might need to take a different route.
What is Part O of the Building Regulations?
Part O (Overheating) came into force in June 2022. It applies to all new residential buildings in England — houses, flats, care homes, student accommodation — and requires developers and designers to demonstrate the building won’t overheat.
There are two routes to compliance: the Simplified Method (Section 1) and Dynamic Thermal Modelling (Section 2), which follows CIBSE TM59. Most standard suburban houses with straightforward glazing can use the Simplified Method. The TM59 route is needed when the Simplified Method fails, when windows have to be kept closed for acoustic reasons, or when the designer wants more flexibility with the glazing design.
What the Simplified Method checks
The Simplified Method checks three things: how much glazing the building has, whether there is enough ventilation to offset it, and whether windows are safe to open.
1. Glazing limits
The assessment measures only the glass area — frames are excluded. The most heavily glazed façade orientation determines the maximum glazing allowance for the whole dwelling. There is also a separate check on the most glazed individual room.
Whether the building has cross-ventilation affects the allowance. A building has cross-ventilation when habitable rooms can be ventilated from openings on two different orientations — broadly, when windows can be opened on opposite or adjacent sides of the dwelling. If that’s not possible, lower limits apply.
2. Ventilation
To offset the solar gain through the glazing, the building must achieve a minimum ventilation free area (equivalent area) of 9% of the total floor area. Bedrooms have their own separate requirement: 4% of the bedroom floor area. This is the actual opening area of the windows — not the frame size.
3. Window sill heights and guarding
- Upper floor windows (means of escape): minimum sill height 1,000mm above finished floor level. Below this, internal guarding is required.
- Upper floor windows (no means of escape): minimum sill height 1,100mm above finished floor level. Below this, internal guarding is required.
Glazing limits by orientation — what the tables say
Part O sets maximum glazing allowances based on three factors: the orientation of the most heavily glazed façade, whether the building has cross-ventilation, and whether the site is in a high or moderate risk location.
High risk locations are specific London postcodes and central Manchester (Appendix C, Approved Document O). The rest of England — including the East Midlands and most of the UK — is classified as moderate risk.
Table 1.1: Limiting solar gains — buildings with cross-ventilation
| Largest glazed façade | High risk: max glazing (% floor area) | High risk: max glazing in most glazed room (% room floor area) | Moderate risk: max glazing (% floor area) | Moderate risk: max glazing in most glazed room (% room floor area) |
|---|---|---|---|---|
| North | 15% | 37% | 18% | 37% |
| East | 18% | 37% | 18% | 37% |
| South | 15% | 22% | 15% | 30% |
| West | 18% | 37% | 11% | 22% |
Table 1.2: Limiting solar gains — buildings without cross-ventilation
| Largest glazed façade | High risk: max glazing (% floor area) | High risk: max glazing in most glazed room (% room floor area) | Moderate risk: max glazing (% floor area) | Moderate risk: max glazing in most glazed room (% room floor area) |
|---|---|---|---|---|
| North | 15% | 26% | 15% | 26% |
| East | 11% | 18% | 18% | 26% |
| South | 11% | 11% | 15% | 15% |
| West | 11% | 18% | 18% | 11% |
What this means for a typical new house — a 100m² example
For a house with a total floor area of 100m² in a moderate risk location, the maximum permitted glazing areas are as follows. The table shows the total glazing limit across the whole dwelling for each façade orientation.
| Largest glazed façade | With cross-ventilation: max total glazing | Without cross-ventilation: max total glazing |
|---|---|---|
| North | 18m² (18% of floor area) | 15m² (15% of floor area) |
| East | 18m² (18% of floor area) | 18m² (18% of floor area) |
| South | 15m² (15% of floor area) | 15m² (15% of floor area) |
| West | 11m² (11% of floor area) | 18m² (18% of floor area) |
Note: each orientation also has a separate limit on the most glazed individual room — see Tables 1.1 and 1.2 above for those figures.
The west-facing column under cross-ventilation rules is often the most restrictive: just 11m² of total glass for a 100m² house. That’s a tight allowance once you factor in living room windows, front door glazing, and any rooflights. For a west-facing dwelling without cross-ventilation, the allowance actually increases to 18m² — which illustrates why ventilation strategy and glazing design have to be considered together.
What counts as glazing area — and what doesn’t
One of the most common misunderstandings is what is actually being measured. The Part O assessment includes only the clear glass area — the transparent surface within the opening. Frames, glazing bars, solid spandrel panels, and any non-transparent elements within a window or door unit are excluded.
This matters particularly for bifold doors and large glazed screens, where the frame proportion can be significant. Measure only the glass, not the overall unit dimensions. For a bifold door system, you are measuring the sum of each individual glass pane — not the overall opening width multiplied by the height.
What the Simplified Method cannot take into account
The Simplified Method has firm limits on what can be credited as a mitigation measure. The following are not permitted:
- External shading — fixed or moveable external shading devices (brise soleil, awnings, shutters, overhangs) cannot be taken into account.
- MVHR — mechanical ventilation with heat recovery is not recognised as a compliance measure under this method.
- Comfort cooling — air conditioning and active cooling systems cannot be counted towards compliance.
- Trees and planting — landscaping, trees, and soft planting cannot be considered under either the Simplified Method or the full TM59 Dynamic Simulation Assessment.
If any of these are the only realistic way to make a scheme comply, TM59 dynamic simulation is the route to explore. External shading in particular can make a material difference in a full TM59 model — and it’s one of the main reasons designers choose the dynamic route when the Simplified Method won’t work.
When do you need TM59 instead?
TM59 is required — or strongly worth considering — when:
- The building fails the Simplified Method’s glazing or ventilation limits
- The site has noise constraints that mean bedroom windows must be kept closed at night
- The building is in a high risk area as identified in Table C1 (you can still assess using the Simplified Method, but there is a low probability of passing)
- The scheme is Passivhaus or very highly insulated and airtight
- The designer wants more flexibility than the Simplified Method’s fixed allowances permit — for example, to credit external shading or solar control glazing
TM59 uses dynamic computer simulation (we use DesignBuilder with EnergyPlus) with a location-specific Design Summer Year (DSY1) weather file. It’s more involved, but it opens up more design options — particularly for schemes with larger glazing areas, external shading, challenging orientations, or acoustic constraints.
Frequently asked questions
Does Part O apply to extensions?
No. Part O applies to new dwellings only. Extensions to existing homes are not subject to Part O. That said, a large south-facing glazed extension can create significant overheating risk — and the Simplified Method principles are a useful sense-check even where the regulation doesn’t formally apply.
Can I use the Simplified Method if my house has bifold doors?
Yes, but the glass area of each panel counts towards the glazing total. Large bifold door arrays facing south or west can push the overall glazing percentage over the limit. If the total exceeds the allowance, the options are to reduce the glazed area or move to TM59 — where external shading and solar control glazing can be modelled and credited.
What happens if my design fails the Simplified Method?
First, establish why — whether it’s the total glazing area, the individual room check, or the ventilation free area. Often a targeted reduction in glazed area, a change to window sizing, or an adjustment to opening dimensions will resolve it. If not, TM59 is the alternative route and can frequently accommodate designs that the Simplified Method cannot.
What is cross-ventilation, and does my house have it?
Cross-ventilation means habitable rooms can be ventilated from openings on two different orientations — broadly, windows on opposite or adjacent façades. A house with windows front and rear typically has cross-ventilation. A flat with openings on one side only does not. The distinction affects which glazing limits apply (Table 1.1 vs Table 1.2), so it’s worth confirming early in the design process.
Can internal blinds help my house pass Part O?
No. Internal blinds, curtains, and internal shading devices cannot be credited under either the Simplified Method or TM59. External shading can be credited under TM59 but not the Simplified Method.
How is the “most glazed façade” determined?
Add up the total glass area on each elevation separately and identify which orientation has the largest total glazed area. That orientation then determines the glazing limits that apply to the whole dwelling — for both the total glazing check and the individual room check.
Find out more or commission an assessment
Low Carbon Box carries out Part O Simplified Method assessments and TM59 Dynamic Simulation overheating assessments for architects, developers, housing associations, and self-builders across England. If you’re unsure which route applies to your project, or want a quick check before you commit to a glazing design, get in touch.
View our overheating assessment services →
Low Carbon Box are accredited On-Construction Domestic Energy Assessors (OCDEA) providing Part O overheating assessments — Simplified Method and TM59 Dynamic Simulation — for homeowners, architects and contractors across the UK. Assessor reference EES/011387.
For more information or to discuss your project: info@lowcarbonbox.co.uk | 01858 322011
