This article covers England and the 2021 edition of Approved Document O. In April 2026 the government announced a review of the document. Until a revised edition is published, the 2021 edition still applies.
Which buildings Part O covers
Part O applies to new residential buildings only (Approved Document O, paragraph 0.3):
- Dwellings: houses and flats.
- Institutional: care homes and similar buildings where people sleep on the premises.
- Other residential: student accommodation, halls of residence and similar.
Requirement O1 itself excludes rooms in a hotel. Shared communal rooms and corridors in buildings with more than one unit are in scope (paragraph 0.4). In a mixed-use building, only the residential parts and the corridors serving them are covered (paragraph 0.7).
How the simplified method works: five checks
1. Find the risk category
| Location | Category |
|---|---|
| The London postcodes listed in Appendix C, Table C1 | High risk |
| The rest of England | Moderate risk |
| Central Manchester (M1, M2, M3, M5, M15, M16, M50) | Moderate risk, but Appendix C says to consider the high-risk guidance |
Check the postcode against Table C1 before any glazing is drawn. The high-risk list covers most of inner and much of outer London, not just the centre.
2. Decide if each unit has cross-ventilation
Cross-ventilation means openings on opposite façades (paragraph 1.4). A corner flat with windows on two adjacent walls does not count. In a block of flats, each unit, communal room and common space is categorised on its own, not the building as a whole.
3. Check the glazing area against Table 1.1 or 1.2
Two limits apply, both set by the orientation of the façade with the largest area of glazing (paragraph 1.6):
- total glazing as a percentage of the unit's floor area, and
- glazing in the most glazed room as a percentage of that room's floor area.
Table 1.1: units with cross-ventilation (maximum glazing area, %)
| Largest glazed façade | High risk: unit | High risk: most glazed room | Moderate risk: unit | Moderate risk: most glazed room |
|---|---|---|---|---|
| North | 15 | 37 | 18 | 37 |
| East | 18 | 37 | 18 | 37 |
| South | 15 | 22 | 15 | 30 |
| West | 18 | 37 | 11 | 22 |
Table 1.2: units without cross-ventilation (maximum glazing area, %)
| Largest glazed façade | High risk: unit | High risk: most glazed room | Moderate risk: unit | Moderate risk: most glazed room |
|---|---|---|---|---|
| North | 15 | 26 | 18 | 26 |
| East | 11 | 18 | 18 | 26 |
| South | 11 | 11 | 15 | 15 |
| West | 11 | 18 | 11 | 11 |
The definitions in Appendix A matter:
- Glazing area is the transparent area only, not the frame.
- Floor area is the gross internal area of the unit.
- Floor area of the room: for an open-plan room, assess the part with the largest glazing area, and take the room depth as no more than 4.5 m from the glazed façade. A deep open-plan kitchen and living room gets no credit for the floor area beyond 4.5 m.
4. Add shading in high-risk locations
In the high-risk part of London, glazing facing anywhere from north-east round through south to north-west also needs one of the following (paragraph 1.9):
- external shutters with a means of ventilation
- glazing with a g-value of 0.4 or less and a light transmittance of at least 0.7
- overhangs with a 50° altitude cut-off, on due-south façades only
5. Check the free area of openings against Table 1.3 or 1.4
| High risk, cross-ventilated | Moderate risk, cross-ventilated | High risk, not cross-ventilated | Moderate risk, not cross-ventilated | |
|---|---|---|---|---|
| Total free area for the unit: the greater of | 6% of floor area or 70% of glazing area | 9% of floor area or 55% of glazing area | 10% of floor area or 95% of glazing area | 12% of floor area or 80% of glazing area |
| Each bedroom | 13% of the room's floor area | 4% of the room's floor area | 13% of the room's floor area | 4% of the room's floor area |
The free area is the geometric open area of the opening. The equivalent area of each opening must be at least its free area, measured to BS EN 13141-1 or calculated using Appendix D (paragraph 1.12). These free areas are usually well above the purge ventilation in Approved Document F.
Worked example: a single-aspect flat outside London
A 70 m² two-bedroom flat in Leeds. All windows are on the south-facing façade, so there is no cross-ventilation and Table 1.2 (moderate risk, south) applies. The living and kitchen area is 6 m wide and 6 m deep.
| Check | Limit | Design | Result |
|---|---|---|---|
| Glazing, whole flat | 15% × 70 m² = 10.5 m² | 9.8 m² of glass | Pass |
| Glazing, most glazed room | 15% × (6 m × 4.5 m) = 4.05 m² | 4.6 m² of glass in the living room | Fail |
| Total free area | Greater of 12% × 70 = 8.4 m² and 80% × 9.8 = 7.84 m², so 8.4 m² | 3.2 m² of openable area | Fail |
| Bedroom free area (12 m² bedroom) | 4% × 12 = 0.48 m² | 0.6 m² | Pass |
Two things come out of it:
- The 4.5 m depth rule bites in open-plan rooms. The living room is 36 m², but only 27 m² counts, so the room limit drops from 5.4 m² to 4.05 m² of glass.
- The total free area is the harder check for single-aspect flats. 8.4 m² of free area means very large opening lights or ventilation louvres. Most single-aspect designs can't provide it without changing the elevations.
The usual options are to cut the glazing in the living room, add openable area or louvres, or model the flat to TM59. TM59 gives credit for things the simplified method ignores, such as the actual window reveals, balconies and neighbouring buildings.
When the simplified method can't be used, or won't pass
Section 3 of Approved Document O sets limits on which openings can count. These often rule the simplified method out even when the glazing passes.
| Situation | What Approved Document O says | Effect |
|---|---|---|
| Noisy site | Windows are likely to be closed at night if bedroom noise exceeds 40 dB L<sub>Aeq,T</sub> over 8 hours, or 55 dB L<sub>AFmax</sub> more than 10 times a night, between 11pm and 7am (paragraph 3.3) | Free areas can't be met with closed windows. Paragraph 3.1 says to use dynamic modelling |
| Ground floor or easily accessible bedrooms | Only openings that can be opened securely count at night (paragraph 3.6) | Needs lockable louvred shutters, grilles or railings, or a smaller free area |
| Openings wider than 100 mm | Handles on outward-opening windows no more than 650 mm from the inside face of the wall; guarding to Table 3.1 that children can't easily climb (paragraph 3.9) | Large openings may need guarding, or smaller windows and more of them |
| Communal heating with long horizontal pipe runs | The simplified method is not suitable (paragraph 1.13) | TM59 modelling, with the pipework heat gains in the model |
| Highly insulated, very airtight or heavily shaded buildings | Paragraph 2.2 says dynamic modelling may give more design flexibility | TM59 may pass a design the simplified method fails |
"Easily accessible" is defined in Appendix A: any part of the window or door within 2 m vertically of an accessible level surface, or a window within 2 m of a flat or low-pitched roof that is within 3.5 m of the ground.
What changes with TM59 modelling
The dynamic thermal modelling route follows CIBSE TM59, with limits set by Approved Document O, paragraph 2.6:
- Daytime (8am to 11pm), occupied rooms: windows start to open at 22°C and are fully open above 26°C.
- Night (11pm to 8am): openings are modelled fully open only if they are on the first floor or above, not easily accessible, and the room is above 23°C at 11pm.
- Ground floor or easily accessible rooms when unoccupied: open in the day only if they can be secured, closed at night.
- Entrance doors: shut all the time.
Internal blinds, curtains and trees can't be counted (paragraphs 2.8 and 2.9). Mechanical cooling may be used only once all practicable passive measures have been tried (paragraph 2.11).
Approved Document O calls for the 2017 edition of TM59. CIBSE published a new edition in 2026, but the 2017 edition is still the one for Part O.
Before you submit: a checklist
- Check the site postcode against Appendix C, Table C1.
- Mark which units have openings on opposite façades.
- Measure glass areas, not window sizes, for each unit and its most glazed room.
- Cap open-plan room depths at 4.5 m from the glazed façade.
- Schedule the free and equivalent area of every opening, using manufacturers' data or Appendix D.
- Ask whether the planning stage raised noise. If it did, plan on TM59.
- Check ground-floor bedroom openings for security, and openings over 100 mm for guarding.
- Complete the Appendix B checklist for building control, and give the occupants the overheating information required by regulation 40B.
More on the service: Building Regulations Part O overheating assessments and CIBSE TM59 overheating assessment.
