Building Regulations Part O explained

Published 4 September 2026 — reviewed against the regs as they stand.

Quick answer: Part O (Overheating) applies to every new residential building in England notified since 15 June 2022 — not to extensions or conversions. A home complies either by the simplified method (glazing capped at 11–18% of floor area depending on orientation, location and cross-ventilation, plus a minimum free area of openings) or by dynamic thermal modelling to CIBSE TM59, which is what you need when the glass exceeds the tables or the windows cannot be assumed open at night — the 40 dB bedroom-noise rule is the usual trigger. Mechanical cooling comes last, after every passive measure.

What Part O is

For decades the Building Regulations cared about keeping heat in. Requirement O1, added to Schedule 1 in 2021 and in force from 15 June 2022, is the first to care about keeping it out. It says that reasonable provision must be made in a new residential building to limit unwanted solar gains in summer and to provide an adequate means to remove heat from the indoor environment. Approved Document O is the government’s guidance on how to do that, and it is a short document with two routes through it.

The trigger was straightforward: better-insulated, more airtight homes with large areas of glass were overheating, and the 2022 heatwaves made the point. Part O sits alongside Part L rather than inside it — a home has to pass both — and it is assessed separately, by an overheating assessment rather than a SAP calculation.

Who it applies to

New residential buildings only. New dwellings, and new institutions and other buildings with rooms for residential purposes — care homes, student halls, boarding accommodation — but not hotel rooms. A conservatory built as part of a new home is inside the requirement; one added later is not. Extensions, loft conversions and a material change of use — an office turned into flats, a barn into a house — are outside it altogether, however glassy they are. That surprises people, because it is precisely the conversions with floor-to-ceiling glazing that overheat; but the regulation is written around buildings that are erected.

Part O also does not apply in Wales or Scotland in this form (see below), and it is not retrospective.

The two routes

The simplified method (Section 1 of Approved Document O) is a check you can do on the drawings: is the glazing under a limit, and do the windows and other openings give enough free area to purge heat? No modelling, no software, no weather file. If a design meets the limits, that is the evidence.

Dynamic thermal modelling (Section 2) is a computer model of the dwelling — its fabric, glazing, shading, ventilation and occupancy — run through a design summer to the CIBSE TM59 methodology, which sets the pass criteria. It costs more and takes longer, but it can pass designs the simplified method cannot, because it credits what the simplified method ignores: thermal mass, external shading, secure night ventilation, the real orientation of every window.

The assessor chooses the route, and the choice is usually made for them by three questions: where is the site, which way does the glass face, and can the windows be left open at night?

The simplified method, step by step

1. Location

Approved Document O splits England in two. A high-risk location is one of the urban and suburban London postcode districts listed in Appendix C, where night-time temperatures stay high and the urban heat island bites; everywhere else is a moderate-risk location. The official FAQ adds that central Manchester also has elevated night temperatures — its minimum standard is the moderate-risk one, but designers are pointed at the high-risk figures for the postcodes it lists.

2. Cross-ventilation

A dwelling is cross-ventilated when it has openings on opposite façades, so air can be driven through it. A house usually is; a single-aspect flat is not; a corner flat with windows on two adjacent walls does not count either. Cross-ventilation earns more generous glazing limits and lower free-area requirements, because the heat has somewhere to go.

3. The glazing limit

Find the largest glazed façade and its orientation, to the nearest compass point (if it sits exactly between two, use the stricter). Then read the maximum glazing area from Table 1.1 or 1.2 — as a percentage of the floor area of the dwelling, with a second, tighter cap on the most glazed room as a percentage of that room’s floor area. Glazing means the transparent area — glass, not frames.

Table 1.1 — cross-ventilated (Approved Document O, 2021 edition):

Table 1.2 — not cross-ventilated:

Two things jump out. West is the enemy: low afternoon sun arrives when the house is already warm, which is why a west-facing, single-aspect flat is capped at 11% of its floor area. And some high-risk figures are more generous than the moderate-risk ones — because in a high-risk location the glazing also has to be shaded (next step), and the tables assume it is.

For scale: a 90 m² house with 18% glazing has 16 m² of glass in total, and a 20 m² living room capped at 37% can have 7.4 m² of it. A south-facing rear elevation with bifolds across the full width will usually blow through the room cap before it troubles the whole-dwelling figure.

4. Shading (high-risk locations)

In a high-risk location, glazing facing anywhere between north-east and north-west through south — in other words everything but a northerly aspect — must be shaded: external shutters that still allow ventilation, glazing with a g-value no higher than 0.4 and a light transmittance of at least 0.7, or, for south-facing glazing only, an overhang that cuts the sun off entirely once it reaches 50° above the horizon. Internal blinds do not count.

5. Free area for removing heat

Limiting the gain is half the requirement; the other half is getting rid of what still comes in, and the simplified method does that with minimum free areas — the actual open area of windows and other openings when opened as designed (Appendix D shows how to work it out from the opening angle), not the size of the frame.

Note the trap: the free-area requirement is tied to the glazing area, so a design that maximises glass right up to the limit needs a great deal of it to open — and a window that only opens 100 mm on a restrictor provides a small fraction of its frame size as free area.

What fails the simplified method

The same things every time: a glazed rear elevation facing south or west; single-aspect flats, especially in London; fixed glazing and picture windows that add to the glazing total without adding any free area; and bedrooms with one small opening. And then there is the case where the numbers pass on paper but the assumption behind them does not hold — which is the next section.

Noise, security and the windows you cannot open

The simplified method’s free areas assume the windows are open at night. Approved Document O says plainly that they will not be if bedroom noise with them open would exceed 40 dB L_Aeq averaged over 11pm to 7am, or 55 dB L_AFmax more than ten times a night — a main road, a railway, a pub garden. It says the same where openings cannot be left open securely: ground-floor bedrooms and any window reachable from a flat roof or balcony, and where outdoor air quality means the ventilation strategy has to limit intake under Part F.

When any of those apply, the simplified method cannot honestly be used for the affected rooms. The route is dynamic modelling, with the windows in the model behaving as they will in life — closed at night in the noisy bedroom — and the design made to pass anyway, through shading, acoustic ventilators, secure louvres, mechanical ventilation with a summer bypass, or, last of all, cooling. This is where most Part O effort and money goes on urban sites, and it is why a noise survey often arrives on the desk before the overheating assessment does.

The dynamic route: what TM59 asks for

A TM59 assessment builds a thermal model of the dwelling — every room, its glazing and orientation, the fabric and its thermal mass, the shading, the ventilation openings and their real free areas — and simulates a whole year with TM59’s occupancy and internal-gain profiles, in the CIBSE Design Summer Year weather file for the 2020s, high emissions, 50th percentile. Windows open in the model when the room passes 22 °C and are fully open at 26 °C; at night they open only where the room is above 23 °C at 11pm and the window is above ground floor and not easily accessible.

The home passes if it meets both criteria:

• Living rooms, kitchens and bedrooms: the operative temperature exceeds the adaptive comfort threshold by 1 K or more for no more than 3% of occupied hours from May to September.
• Bedrooms at night: the operative temperature between 10pm and 7am exceeds 26 °C for no more than 1% of annual hours — 32 hours a year.

The bedroom criterion is the one that fails, and it fails on west-facing bedrooms with a single window first. The fixes, in the order an assessor reaches for them: cut or shade the glazing, improve the g-value, add secure night ventilation, use cross-ventilation where the plan allows, add thermal mass, add mechanical ventilation with a boost — and only then cooling, because Requirement O1 makes mechanical cooling the last resort and Approved Document O asks the designer to show Building Control that all practicable passive means were used first. Cooling that is installed also has to be carried into the Part L calculation, where it costs energy.

CIBSE published a substantially revised TM59 in July 2026 — a passive-first three-stage method, four criteria and a rewritten bedroom test. Approved Document O still cites the 2017 edition, so for Building Control the 2017 criteria remain the test; what the 2026 rewrite changes is worth knowing if you are designing now.

What Building Control gets

For either route: the compliance checklist in Appendix B of Approved Document O, completed and signed, showing which route was used and the figures. For the modelling route, the TM59 report with the contents the methodology specifies. And the owner must be given non-technical information on how the home is meant to stay cool — which windows to open, what the shading is for — because a passive strategy only works if the people in the house know it exists.

Wales, Scotland and London

Wales has its own Part O and Approved Document O, in force since 23 November 2022, with the same two routes but a simplified method organised by single- or dual-aspect dwellings rather than by London postcode. Scotland has no Part O; overheating is Standard 3.28 of the Domestic Technical Handbook, in force since 1 December 2022, again with a simple method and a TM59 route. London adds a planning-stage layer on top of Part O: the London Plan’s cooling hierarchy expects major schemes to demonstrate overheating performance with TM59 modelling at application stage — so a London scheme often models twice, once for planning and once for Building Control. Our Part O service covers all three.

The bottom line

Part O is a drawings problem before it is a modelling problem. If the largest glazed façade faces north or east, the dwelling is cross-ventilated, the site is not in a London high-risk postcode and the bedrooms are quiet enough to sleep with the windows open, the simplified method will usually pass and the assessment is quick. Move any one of those — south or west glass, a single-aspect flat, a main road — and you are into TM59, where the cost of the fix rises the later it is found. The cheapest Part O assessment is the one done on the planning drawings.