Part O Explained: The Overheating Rules Reshaping New Homes in London

Part O is the newest part of the Building Regulations and the one that most reliably ambushes development teams. In force since 15 June 2022, it requires every new residential building in England to demonstrate – by calculation, before construction – that it will not overheat in summer. It has quietly redrawn façades across London: glazing ratios that sailed through planning a few years ago now fail building control, and the fully glazed south-facing living room has become an engineering problem rather than a sales feature. This guide explains what Part O actually requires, how the two compliance routes work, and why most serious London schemes end up in dynamic thermal modelling.

What requirement O1 demands

Requirement O1 has two limbs: limit unwanted solar gains in summer, and provide an adequate means of removing excess heat from the indoor environment. Two conditions sit alongside them. First, the strategy must account for occupant safety and reasonable enjoyment – which is where noise, security and protection from falling enter the calculation. Second, mechanical cooling may only be used where insufficient heat can be removed without it: passive measures first, by regulation. The scope covers new residential buildings – dwellings, plus institutions and other buildings with rooms for residential purposes such as care homes and student accommodation (hotel rooms are excluded). It is also worth being clear about its purpose: Part O protects health and welfare. The approved document states explicitly that following it does not guarantee comfort – a distinction that matters enormously at the prime end of the market, as we return to below.

Moderate risk, high risk: why the postcode matters

Approved Document O divides England into two categories. Everywhere is a "moderate risk" location except a defined list of urban and suburban London postcodes – set out in Appendix C of the document – which are "high risk", reflecting the urban heat island and the density of the capital. The category changes the numbers substantially: high-risk locations face tighter glazing caps, larger openable areas in bedrooms, and a mandatory requirement to shade all glazing facing between north-east and north-west via the south, using external shutters, external blinds, overhangs or awnings. Internal blinds and curtains do not count towards compliance, and neither does tree cover.

Route 1: the simplified method

The simplified method is a set of look-up tables: maximum glazing areas to limit solar gain, and minimum openable "free areas" to remove heat, varying by risk location, by whether the dwelling has cross-ventilation (openings on opposite façades), and by the orientation of the most glazed façade. The caps are tighter than many design teams expect. A cross-ventilated home in a high-risk London postcode with its largest glazed façade facing south is limited to glazing of 15% of total floor area, and 22% of floor area in its most glazed room. Without cross-ventilation – the position of most single-aspect flats – the south-facing caps fall to 11% and 11%. On the heat-removal side, bedrooms in high-risk locations need openable free area of 13% of the room's floor area, against 4% in moderate-risk locations, and the dwelling as a whole must meet totals rising to 95% of the glazing area for non-cross-ventilated homes. Openings must be demonstrated by equivalent-area calculation or test data, not by the hopeful assumption that a window "opens fully".

The usability tests that break the simplified method

Section 3 of the approved document asks a blunt question: will occupants actually be able to use the openings the calculation relies on? Three tests do most of the damage in London. Noise: where external noise is an issue, windows are assumed closed during sleeping hours if bedroom levels would exceed 40dB LAeq averaged over the night (11pm to 7am), or 55dB LAFmax more than ten times a night – thresholds that busy roads, railways and night-time economy areas exceed comfortably. Security: for ground-floor and easily accessible bedrooms, only openings that can be secured – lockable shutters, grilles, railings – count towards ventilation during sleeping hours. Falling: openings above defined heights need guarding and restricted geometry to count at all. The approved document is explicit about the consequence: if these constraints mean the free areas cannot be delivered, the simplified method cannot be used and the scheme must be assessed by dynamic thermal modelling. That single paragraph is why most urban London schemes go straight to modelling.

Route 2: CIBSE TM59 dynamic thermal modelling

The modelling route simulates the building hour by hour through a Design Summer Year – weather data representing a hot summer for the location – with standardised occupancy and internal gains defined by CIBSE's TM59 methodology. Two headline criteria decide the result. Living spaces are assessed against the adaptive comfort limits of CIBSE TM52: hours of overheating beyond the comfort threshold must stay within 3% of occupied hours from May to September. Bedrooms face a fixed test at night: the temperature must not exceed 26°C for more than 1% of annual sleeping hours – roughly 32 hours a year. Approved Document O then constrains the modeller's choices so results cannot be engineered: windows may only be modelled opening progressively between 22°C and 26°C when rooms are occupied; night-time opening may only be assumed for windows on the first floor or above that are not easily accessible, and only when the room is above 23°C at 11pm; ground-floor openings are modelled shut overnight. Marginal façades fail under these rules, which is precisely their purpose.

Where mechanical cooling fits

Part O sets a hierarchy for removing heat: opening windows, ventilation louvres, mechanical ventilation, and only then mechanical cooling. The design team must demonstrate to building control that all practicable passive means have been used before air conditioning is adopted for compliance purposes. But note what this does and does not say. In dense parts of London the passive case genuinely runs out – a single-aspect flat over a main road cannot rely on open windows at night, and the approved document's own noise and security rules say so. In those cases mechanical cooling is a legitimate, documented part of the compliance strategy. Separately, on prime residential projects, clients expect comfort in a heatwave, not merely the absence of a health hazard – and Part O explicitly does not promise comfort. The engineering answer is to do both properly: a fabric and shading strategy that passes Part O on its own merits, with quiet, zoned comfort cooling engineered as a quality layer above the regulatory baseline, and ventilation designed as a system rather than an afterthought.

What Part O means for the rest of the design

Part O never acts alone. It pulls against Part L, which rewards winter solar gain that Part O penalises in summer – the same window is an asset in one calculation and a liability in the other, and only whole-building thinking resolves the trade. It leans on Part F, because purge ventilation and background ventilation must still be delivered, sometimes at larger rates than Part F alone would require. And it reaches into acoustics, because the windows-closed decision turns on a noise assessment that is cheap at RIBA Stage 2 and painful at Stage 4. On our projects the overheating strategy, the ventilation strategy and the cooling strategy are one coordinated piece of engineering – the approach we set out in our complete guide to MEP design for prime residential projects.