Approved Document O explained
A working guide to overheating in new homes in England: which buildings Part O covers, when the simplified method works and when you need TM59 modelling, and the noise and security rules that decide whether windows can be relied on.
What is Approved Document O?
Approved Document O is the statutory guidance for Part O of the Building Regulations 2010, which requires new residential buildings in England to limit unwanted solar gain in summer and provide an adequate means of removing heat. Compliance is shown either with the simplified method in Section 1 or with dynamic thermal modelling to CIBSE TM59 in Section 2, and in both cases the strategy must still work once noise, security and safety are taken into account.
Requirement O1 in Schedule 1 applies to "a dwelling, institution or any other building containing one or more rooms for residential purposes, other than a room in a hotel". Reasonable provision must be made to "limit unwanted solar gains in summer" (O1(1)(a)) and to "provide an adequate means to remove heat from the indoor environment" (O1(1)(b)).
O1(2) adds two conditions. Account must be taken of "the safety of any occupant, and their reasonable enjoyment of the residence", and mechanical cooling may only be used where insufficient heat can be removed without it. Air conditioning is therefore a last resort: reducing glazing, shading and openable windows come first.
Regulation 40B sits alongside O1. Within five days of completion, the owner must be given enough information to operate the overheating strategy. The guidance suggests a non-technical overview and a "Staying cool in hot weather" section in the home user guide.
An overheating strategy that relies on windows nobody will open at night is not a strategy.
Is Approved Document O changing?
Not yet. The 2021 edition, published on 15 December 2021 and in force since 15 June 2022, is still current in October 2026. In its response to the Future Homes and Buildings Standards consultation, published on 24 March 2026, the government stated it "will proceed with a full technical review of Approved Document O", without a timetable. Industry summaries list an updated TM59, an improved simplified method, noise and security guidance and some material changes of use as topics. Wales has its own Part O, in force since 23 November 2022; see our guide to building regulations in Wales.
Which buildings does Part O apply to?
Part O applies to new residential buildings only. Table 0.1 lists dwellings, institutional and other residential buildings, and the residential parts of other buildings.
Covered: new residential
Houses and flats; homes where people sleep on the premises, including care for older or disabled people; student accommodation, halls and residential colleges.
Not covered
Hotel rooms are excluded by the wording of O1. Extensions and material changes of use are not currently covered.
In mixed-use buildings, the residential parts and the corridors serving them are in scope, as are communal rooms and common spaces in multi-unit buildings.
The government's review will consider extending Part O to dwellings created through some changes of use, but for now a conversion or an extension does not need a Part O assessment.
Simplified method or dynamic thermal modelling?
The two routes do the same job in different ways. The simplified method sets limits on glazing and minimum opening areas from tables; dynamic thermal modelling tests the building against CIBSE TM59 using assumptions fixed by Approved Document O.
The simplified method suits straightforward houses and flats with good cross-ventilation and modest glazing, and the evidence for building control is a set of calculations against the tables. Modelling suits larger glazed schemes, single-aspect flats, noisy sites and communal heating, and the evidence is a report showing the building passes TM59. Modelling has no restriction on use; the simplified method does.
Many flatted schemes go straight to TM59 because of communal heating, single-aspect units or night-time noise. Modelling remains an option even where the simplified method is available.
How the simplified method works
The simplified method is four checks in sequence: the risk location, cross-ventilation, a maximum glazing area and a minimum free area for removing heat.
- 01
Risk location
High risk: the London postcode areas in Appendix C. Moderate risk: everywhere else.
Appendix C - 02
Cross-ventilation
Openable windows on opposite façades. Adjacent façades, as in a corner flat, do not count.
Opposite façades only - 03
Glazing limit
Maximum glazing by location, cross-ventilation and orientation. High-risk, south-facing, no cross-ventilation: 11% of floor area.
Tables 1.1 and 1.2 - 04
Free area
Moderate risk, cross-ventilated: greater of 9% of floor area or 55% of glazing. Bedrooms: 13% high risk, 4% moderate.
Table 1.3 · Appendix D
South- and west-facing units without cross-ventilation have the tightest glazing limits. Each limit is expressed both as a share of the unit's floor area and of the floor area of the most glazed room.
Location and cross-ventilation
Approved Document O splits England into high-risk locations, the urban and some suburban London postcode areas listed in Appendix C, and moderate-risk locations everywhere else. The guidance notes that parts of central Manchester (postcodes M1, M2, M3, M5, M15, M16 and M50) may also have elevated night-time temperatures, and suggests designers consider following the high-risk guidance there.
Glazing and shading
Tables 1.1 and 1.2 set the maximum glazing area. In high-risk locations, glazing facing anywhere from north-east through south to north-west also needs shading, by one of three options: external shutters with a means of ventilation; glazing with a g-value of 0.4 or less and light transmittance of 0.7 or more; or, on due-south façades only, overhangs with a 50° altitude cut-off.
Free area for removing heat
Table 1.3 sets minimum free areas for openings, and each bedroom needs its own. Free area is the geometric open area of the opening, calculated using Appendix D or measured to BS EN 13141-1.
What TM59 modelling under Part O involves
Under Section 2, building control receives a report showing the building passes the CIBSE TM59 overheating assessment. Approved Document O then fixes how the model is set up, so results cannot be flattered by optimistic assumptions.
TM59 sets the pass criteria: broadly, an adaptive comfort test for living rooms, kitchens and bedrooms, and a fixed night-time test that bedroom temperatures between 10pm and 7am should not exceed 26°C for more than 1% of annual hours.
Entrance doors are always modelled shut. The ground-floor row is often what pushes a scheme towards mechanical ventilation.
Shading is fixed too. Internal blinds, curtains and planting cannot be counted, whatever the modelling software allows. Fixed shading, glazing design, g-value, reveals and shading from adjacent buildings can be. Mechanical cooling may only be modelled where the requirement cannot be met by opening windows and other passive measures.
How noise, security and safety affect Part O
Section 3 applies to both methods, and it is where many overheating strategies fail. A window only counts if it can realistically be open when heat needs removing.
- 40dB
Noise at night
Windows assumed closed 11pm to 7am if bedrooms exceed 40 dB LAeq,T over 8 hours or 55 dB LAFmax more than 10 times a night.
Busy roads · rail - ADF
Pollution
Openings near significant pollution sources designed in line with Section 2 of Approved Document F.
Opening position - 2m
Security
Ground-floor or easily accessible bedrooms: only openings secured while open count, such as lockable louvred shutters or fixed grilles.
Within 2m of a surface - 1100
Falling
Guarding to Table 3.1 where there is a drop; 1,100 mm for full-height openings where the drop exceeds 600 mm.
Guarding height mm - 100
Entrapment
Louvres, grilles and railings must not pass a 100 mm sphere. Looped blind cords need child safety devices.
Sphere test mm
"Easily accessible" generally means within 2 m vertically of an accessible surface. On busy roads or near rail lines, the noise test usually means modelling closed windows and removing heat another way, such as enhanced mechanical ventilation.
Common Part O problems at design stage
- Test Part O before the façade is fixed. Glazing ratios agreed at planning can make the simplified method impossible to pass.
- Check for communal pipework. Horizontal heating distribution in corridors makes the simplified method unavailable and adds heat to common areas.
- Do not rely on windows that will be shut. Agree noise, security and child safety assumptions early with the acoustician and the client.
- Do not count internal blinds. They do not count under Approved Document O.
Where Avoice fits
Part O compliance depends on decisions made by the architect, the energy assessor, the acoustician and the client, often months apart. The risk is a route chosen at Stage 2 that no longer holds by Stage 4.
The Avoice Regulations library has the full text of Approved Document O for England and Wales alongside Parts F and L, with AI search that cites whether an answer comes from legislation or guidance. The Building Regs Tracker in Avoice Practice then records which compliance route a project is using and which evidence, such as the TM59 report, is still outstanding.
Software can retrieve, compare and flag. The energy assessor, the architect and building control still decide whether the strategy works and whether the evidence is adequate.
Search Approved Document O with citations
Approved Document O for England and Wales, alongside Parts F and L, in one library with answers that cite their source.
This guide is for general education and is not professional advice. Check the current Approved Document on GOV.UK, and the version that applies in Wales, before relying on any paragraph.