Part O of the Building Regulations has governed overheating in new homes across England since it came into force on 15 June 2022, and it divides the country into moderate-risk and high-risk locations for summer heat (PBC Today, 2022). For installers and specifiers working on new build and change-of-use projects, it reshapes how glazing, shading and ventilation are designed long before the first solar panel or heat pump is fitted.
The rule exists because better insulated, more airtight homes trap heat in summer, a pressing problem given that UK buildings account for around 40% of national emissions and homes for roughly 14% (Partel, 2025). This guide explains what Part O requires, the two routes to compliance, the overheating assessment thresholds, and where all of it meets solar gain and low-carbon heating.
Key Takeaways
- Part O applies only to new residential buildings: houses, flats, care homes, student halls and living accommodation for children aged five and over (Partel, 2025).
- Compliance follows one of two routes: the simplified method, or dynamic thermal modelling to CIBSE TM59 (GOV.UK, 2022).
- England is split into moderate-risk and high-risk overheating areas, with high-risk covering urban and some suburban parts of London (Partel, 2025).
- A TM59 assessment caps bedroom temperature at 26C for no more than 1% of night hours, about 32 hours a year (BuildComply, 2025).
- Overheating strategy must be handed to occupants in a Home User Guide, and passive measures come before mechanical cooling (PBC Today, 2022).
What is Part O of the Building Regulations?
Part O is the overheating section of the Building Regulations for England, published on 15 December 2021 and in force from 15 June 2022 (PBC Today, 2022). It requires new residential buildings to limit unwanted solar gain in summer and to provide a means of removing excess heat, protecting the health and welfare of the people who live there.
The regulation grew out of a decade of evidence gathered by CIBSE and the Building Regulations Advisory Committee, who warned that an energy strategy focused only on winter heat loss had left new homes exposed to summer overheating (PBC Today, 2022). As fabric standards tightened and heatwaves became more frequent, overheating moved from a comfort issue to a health one, and Approved Document O is the guidance that shows how to meet the new requirement in practice.
Part O also looks at whether an overheating strategy is actually usable. It asks designers to consider noise, air pollution and security near the home, because a window that residents will not open at night because of traffic noise or safety concerns does nothing to remove heat (Partel, 2025). That usability test is where paper compliance and real-world comfort often diverge, and it is worth flagging to clients early.
Which buildings does Part O apply to?
Part O applies only to new residential buildings. That covers dwellings such as houses and flats, institutions where people sleep, and other residential accommodation including student halls and living space for children aged five and over (Partel, 2025). It does not cover existing homes unless a material change of use creates a new dwelling.
For an installer, the boundary matters. A standard retrofit, an extension or a loft conversion on an existing house sits outside Part O, though the same physics still applies and good overheating practice still protects the client. New build plots and conversions that create new dwellings, on the other hand, must show a Part O strategy as part of the wider planning and building control process. Getting that clear at survey stage avoids awkward redesigns once glazing and roof layout are set.
The two routes to Part O compliance
Approved Document O sets two ways to demonstrate compliance: a simplified method built on fixed glazing and ventilation limits, and dynamic thermal modelling to CIBSE TM59 (GOV.UK, 2022). Most complex, urban or highly glazed schemes use modelling, because it offers more design flexibility than the simplified route and can target specific rooms rather than the whole building.
The choice usually comes down to how far the design pushes glazing and how much flexibility the team needs. The comparison below sets out the practical trade-offs:
- Simplified method: works from prescribed maximum glazing areas and minimum openable free areas by orientation, risk zone and cross-ventilation. Best for straightforward, well-ventilated, low-to-moderate risk homes.
- Dynamic thermal modelling (TM59): simulates the home hour by hour against standard weather and occupancy data. Best for urban, single-aspect, highly glazed or unusual designs where the simplified limits are too tight.
- Flexibility: the simplified route is rigid but quick to check; TM59 is more work but lets designers justify larger glazing with shading or ventilation offsets.
- Effort: the simplified method needs window-by-window glazing and opening calculations; TM59 needs specialist software and a competent assessor.
How does the simplified method limit glazing?
The simplified method caps glazed area on the east, south and west facades and sets a minimum openable free area to remove heat (PBC Today, 2022). Limits tighten for single-aspect homes without cross-ventilation and for high-risk locations, where south-facing glazing can fall to roughly 11% to 15% of floor area depending on the layout (Centre for Cities, 2024).
Cross-ventilation, defined as openings on opposite facades that let air flow through, is the single biggest lever. A cross-ventilated home earns a more generous glazing allowance and a lower minimum free area than a single-aspect flat with windows on one side only (PBC Today, 2022). Feedback from industry is that applying the simplified method is anything but simple, so it tends to suit one-off homes in low-to-moderate risk areas rather than dense apartment schemes.
The method also demands window-by-window detail. The designer has to check the glazed area of every opening against the facade limit, then confirm the openable free area is enough to purge heat, taking the larger figure of the glazing-based and floor-area-based requirements (PBC Today, 2022). In practice, a scheme that fails the simplified check on glazing alone does not have to be redesigned from scratch: it can move to the modelling route instead, which is why many teams start with the simplified method as a quick sanity check.
What does a TM59 overheating assessment check?
A TM59 assessment models the home against CIBSE's 2020s Design Summer Year weather and tests two criteria (PBC Today, 2022). Living rooms and bedrooms must stay within the adaptive comfort limit for at least 97% of occupied hours, and bedrooms must not exceed 26C for more than 1% of night hours between 10pm and 7am (BuildComply, 2025).
That 1% works out at about 32 hours a year, so 33 hours or more above 26C in a bedroom is a fail (Overheating Expert, 2025). The dwellings most at risk are single-aspect flats, top-floor units, and homes with large unshaded east, south or west glazing, which is exactly where assessors focus. Approved Document O expects overheating to be solved by passive means as far as reasonably practical, with mechanical cooling used only when other options are exhausted (PBC Today, 2022).
The adaptive comfort limit that Criterion A relies on comes from CIBSE TM52, which sets a running comfort temperature rather than a single fixed number, so the threshold shifts with recent outdoor conditions (BuildComply, 2025). Because the model runs against future-weighted 2020s weather data, a design that only just passes today has little headroom as summers warm, which is a fair point to raise with clients who want the largest possible glazing.
How Part O affects solar, shading and low-carbon homes
Part O reshapes facade design in exactly the homes where installers fit solar and heat pumps. Large south and west glazing that boosts winter solar gain also drives summer overheating, so shading, glazing specification and ventilation have to be balanced against panel layout and available roof space (Partel, 2025). Where Part O demands more purge ventilation than Part F, the higher standard applies.
On airtight new builds we increasingly see external shading, brise soleil and solar-control glazing specified up front, and those choices interact directly with roof-mounted PV positioning, mounting zones and MVHR routing. A careful shading analysis at design stage stops overheating mitigation and panel yield from fighting each other. It pays to flag these constraints during the survey rather than discovering them when building control reviews the Part L and Part O evidence together.
There is also a direct overlap with ventilation and escape. Where Part O needs a bigger purge opening than Part F to shift heat, the higher of the two standards applies, and where an escape window under Part B adds glazing, that extra glass counts against the overheating calculation (Partel, 2025). For the installer, the lesson is that window size, purge ventilation, fire escape and solar gain are one linked problem, not four separate ones, and the earlier they are reconciled the cheaper the build stays.
Part O, the Future Homes Standard and what comes next
Part O sits alongside Part L and Part F in the push toward the Future Homes Standard, which targets a 75% to 80% cut in new-home carbon and embeds overheating design as a default rather than an afterthought (Partel, 2025). Overheating scrutiny is expected to tighten as later amendments roll through the regulations in 2026 and beyond.
For installers, the practical takeaway is to raise overheating early with the design team, because glazing and shading decisions ripple straight into roof layout, ventilation and system sizing. Tools that keep survey notes, system design and compliance detail in one place, such as Reonic's platform for renewables installers, make it easier to carry those constraints from the first site visit through to handover without losing them along the way.
Frequently asked questions
Does Part O apply to extensions or existing homes?
No. Part O applies to new residential buildings and to work that creates a new dwelling through a material change of use (Partel, 2025). Standard extensions, loft conversions and refurbishments of existing homes fall outside it, although the same overheating physics applies and sensible glazing and shading choices still protect the occupant.
What is the difference between Part O and Part L?
Part L covers energy efficiency and carbon, capping heat loss and setting fabric and system standards, while Part O limits summer overheating. The two can pull in opposite directions, because heavy insulation helps Part L but can trap heat, so a compliant design has to satisfy both together (GOV.UK, 2022).
How many hours above 26C fail a TM59 bedroom check?
TM59 allows a bedroom to exceed 26C for no more than 1% of night hours between 10pm and 7am, which is about 32 hours across the year. Thirty-three hours or more above 26C is a fail, which is why bedroom design gets the closest attention (BuildComply, 2025).
Is mechanical cooling allowed under Part O?
Yes, but only as a last resort. Approved Document O expects overheating to be tackled by passive means first, such as limiting glazing, adding shading and improving ventilation, with mechanical cooling used only where passive measures cannot achieve compliance (PBC Today, 2022). Reliance on air conditioning to pass is discouraged.
Who signs off Part O compliance?
The building control body reviews the Part O evidence, whether that is the simplified method calculations or a full TM59 report, as part of the wider Building Regulations approval (GOV.UK, 2022). The overheating strategy must also be passed to occupants in a Home User Guide so they can operate the home as designed (Partel, 2025).
