Why Part O exists
Approved Document O was introduced to tackle overheating in new homes — a growing problem as dwellings become more airtight and better insulated to meet Part L, and as summers get hotter. Requirement O1 asks two things of a new residential building: limit unwanted solar gain in summer, and provide an adequate means of removing heat from the indoor environment. It came into force in England on 15 June 2022.
It applies to new residential buildings only — houses and flats, care homes and other institutional accommodation, student halls and similar residential rooms, and their shared spaces. It does not apply to extensions, to conservatories bolted on afterwards, or to a change of use. It does not apply to hotels.
Two routes to compliance
The simplified method is a set of limits in tables. First, limit solar gains: a maximum glazing area as a percentage of floor area, set by the orientation of the facade with the most glass, by whether the dwelling is cross-ventilated (openings on opposite facades), and by location — anywhere from around 11% to 18% of floor area depending on those three things, with a separate cap on the most-glazed room. Second, remove excess heat: a minimum free area of openable windows and vents, again by location and cross-ventilation. For a cross-ventilated dwelling outside London that is the greater of 9% of floor area or 55% of the glazing area, with bedrooms needing openings of 4% of the room’s floor area.
It suits straightforward houses and dual-aspect flats with sensible glazing. It is quick and cheap. It also fails a lot of modern designs.
Dynamic thermal modelling builds a model of the dwelling and simulates it through a design summer to CIBSE TM59, the overheating methodology for homes. It accounts for location, solar radiation, ventilation, fabric, thermal mass and shading — and it is the route that finds a pass where the simplified method cannot: bigger glazing, single-aspect flats, and any site where the windows cannot be assumed open.
We look at the drawings, tell you which route the scheme needs, and price that.
High risk means London
Approved Document O defines a high-risk location as the urban and some suburban parts of London listed by postcode district in its Appendix C — broadly central and inner London. Everywhere else in England is moderate risk. Central Manchester gets a note about elevated night-time temperatures but is not a high-risk location for compliance purposes.
One thing that surprises people: some high-risk glazing limits are more generous than the moderate-risk ones. That is not a misprint. High-risk buildings must also provide external shading to any glazing facing between north-east and north-west via south, and the glazing tables already assume it.
The pass criteria under TM59
The dynamic route passes or fails on two criteria from TM59:
- Criterion A: living rooms, kitchens and bedrooms must not exceed their comfort temperature by 1°C or more for more than 3% of occupied hours between 1 May and 30 September.
- Criterion B: bedrooms must not exceed 26°C for more than 1% of the annual hours between 10pm and 7am — 33 hours or more is a fail.
Approved Document O also fixes how the model is allowed to treat the occupants: windows opened gradually between 22°C and 26°C by day, bedroom windows open all night if the room is above 23°C at 11pm, and no internal blinds or curtains assumed. The model cannot pass a building by imagining people who never open a window in the day and always close a blind.
The weather file is the CIBSE Design Summer Year for the 2020s, high-emissions, 50th percentile — TM59’s specification, which Part O adopts by reference.
Noise, security and the windows you cannot open
This is where most schemes end up in modelling, and it is worth understanding because it is where the money goes. The simplified method assumes the windows are open at night. Approved Document O says they will not be if bedroom noise with them open exceeds 40 dB averaged over 11pm to 7am, or 55 dB more than ten times a night — and that where this means the simplified method’s opening areas cannot be achieved, dynamic thermal modelling must be used. Main roads, railways and flight paths put a scheme into TM59 before the architect has drawn a window.
The same logic applies to security — only openings that can be left open securely count towards the free area, which bites on ground-floor and easily reached bedrooms — and to protection from falling, where opening restrictors, handle heights and guarding all reduce what counts. Louvres, grilles and railings must not pass a 100 mm sphere. None of this is difficult, but all of it has to be in the model.
Mechanical cooling comes last
Requirement O1 is explicit: mechanical cooling may only be used where insufficient heat can be removed without it, and Approved Document O requires the building to be shown to Building Control to have used all practicable passive means first — orientation, shading, ventilation, thermal mass. Air conditioning is not banned; the government said so plainly in June 2026. It is the last step, and you have to have tried the others.
Mitigation is usually simpler than people expect: an overhang, external louvres on the west elevation, a different glass, a bigger opening light, a cross-ventilation path that was nearly there anyway.
What Building Control gets, and what the owner gets
For the modelling route, a TM59 report demonstrating the pass, with the contents TM59 specifies, plus the compliance checklist from Appendix B of Approved Document O. For either route, the owner must be given non-technical information on the overheating strategy — what the measures are, where they are, when to use them — within five days of completion, and it goes in the home user guide under “Staying cool in hot weather”.
Wales and Scotland
Wales has its own Part O and its own Approved Document O, in force since 23 November 2022. Same two routes, but no high-risk-location concept: the guidance applies uniformly, and the simplified method is organised by building type and single- or dual-aspect rather than by London postcode. Don’t reuse the English tables on a Welsh site.
Scotland introduced Standard 3.28 on overheating from 1 December 2022, with a simple method and a TM59 route in the Domestic Technical Handbook. We assess to it.
London adds a planning-stage layer on top of Part O: the London Plan’s Policy SI 4 requires major developments to demonstrate overheating performance through the cooling hierarchy with TM59 modelling at application stage. A London scheme typically needs both — see our London SAP and energy compliance page.
What is changing
Not Approved Document O itself — the Future Homes Standard regulations coming into force in March 2027 do not amend it, and there is no 2026 edition. But CIBSE published a revised TM59 in July 2026, with a new bedroom criterion and future weather years; CIBSE’s own position is that TM59 (2017) remains the Part O requirement while the 2026 version is best practice. And the government has committed to a full technical review of Approved Document O following its call for evidence, without a date. We track both and will tell you when the answer changes.
Get it checked early
Overheating problems are cheap to fix on paper and expensive to fix on site. Part O is usually one item on a longer list — see the rest of our residential compliance services. Send us your drawings at design stage — get a quote or call 01202 623236.
Common questions
What is a Part O overheating assessment?
The evidence Building Control needs that a new residential building limits unwanted summer solar gain and can get rid of excess heat — Requirement O1 of the Building Regulations, in force in England since 15 June 2022. It is done one of two ways: the simplified method, which checks glazing area and ventilation opening area against tables in Approved Document O, or dynamic thermal modelling, which simulates the building through a summer to CIBSE's TM59 methodology and reports whether it passes.
Does Part O apply to extensions or conversions?
No. Approved Document O applies to new residential buildings only — houses, flats, care homes and other institutional accommodation, student halls and similar. It does not apply to extensions, to conservatories added after the building is finished, or to a change of use. A conservatory built as part of a new home is included.
What is the difference between the simplified method and dynamic thermal modelling?
The simplified method is a set of limits: maximum glazing area as a percentage of floor area, set by the orientation of the most-glazed facade and by whether the dwelling is cross-ventilated, plus a minimum free area of openings to purge heat. It suits straightforward houses and flats. Dynamic thermal modelling builds a computer model of the dwelling and runs it through a design summer to CIBSE TM59, so it can pass buildings with more glass, single-aspect flats, and anything where the windows cannot be assumed open.
Why does noise push a scheme into dynamic modelling?
Because the simplified method assumes windows are open at night. Approved Document O says that where bedroom noise with the windows open would exceed 40 dB averaged over 11pm to 7am, or 55 dB more than ten times a night, occupants are likely to close them — and if that means the simplified method's opening areas cannot be achieved, dynamic thermal modelling has to be used. Sites near main roads, railways and flight paths usually end up in TM59 for this reason.
What does a Part O assessment need to pass under TM59?
Two criteria. Living rooms, kitchens and bedrooms may not exceed their comfort temperature by 1°C or more for more than 3% of occupied hours between May and September; and bedrooms may not exceed 26°C for more than 1% of the annual hours between 10pm and 7am — 33 hours or more is a fail. Approved Document O fixes how the model treats windows: opened between 22°C and 26°C by day, bedroom windows open all night if the room is above 23°C at 11pm, and no internal blinds or curtains assumed.
Is my site a high-risk location?
Only if it is in one of the London postcode districts listed in Appendix C of Approved Document O — broadly central and inner London. Everywhere else in England is moderate risk. Central Manchester is mentioned as a note but is not a high-risk location for compliance. High-risk sites must also provide shading to glazing facing anywhere between north-east and north-west via south.
Can I just fit air conditioning?
Not as the first answer. Requirement O1 says mechanical cooling may only be used where insufficient heat can be removed without it, and Approved Document O requires you to show Building Control that all practicable passive measures — orientation, shading, ventilation, thermal mass — were used first. Air conditioning is not banned, and the government has said so, but it is the last step, not the first.
What about Wales and Scotland?
Wales has its own Part O and its own Approved Document O, in force since 23 November 2022 — the same two routes, but with no high-risk-location concept and a simplified method organised by single- or dual-aspect rather than by London postcode. Scotland introduced Standard 3.28 on overheating from 1 December 2022 with a simple method and a TM59 route. We assess to all three.
Has Part O changed in 2026?
The document itself has not — the Future Homes Standard regulations coming into force in March 2027 do not touch Part O, and there is no 2026 edition. Two things have moved. CIBSE published a revised TM59 in July 2026, but TM59 (2017) remains what Part O requires. And the government has committed to a full technical review of Approved Document O, with no completion date yet. We track it.