Building Regs Part O (Overheating): What Builders & Developers Need to Know (2026)

What Is Part O and Why Was It Introduced?

Part O of the Building Regulations covers overheating in new residential buildings. It came into force in England in June 2022 (with a transitional period for projects already underway) and is supported by Approved Document O, the official guidance on how to comply.

The requirement has two halves. A new residential building must use means to limit unwanted solar gains in summer, and it must provide an adequate means to remove excess heat from the indoor environment. In plain terms: stop too much heat coming in through glazing, and give occupants a reliable way to get heat back out.

The driver was straightforward. Homes built to modern energy standards are highly insulated and airtight. That keeps them warm in winter, but the same fabric traps heat in summer. As UK summers get hotter, regulators became concerned about heat-related risk in bedrooms in particular — overheating is most dangerous at night, when occupants are asleep and cannot easily respond. Part O exists to make sure new homes do not bake.

Who Does Part O Apply To?

Part O applies to new residential buildings in England. That includes the obvious — new houses and flats — but the scope is deliberately broad and catches building types that are easy to forget:

• New dwellings (houses and flats)
• Residential care homes
• Student accommodation and halls of residence
• Other institutional and similar residential buildings where people sleep

Just as important is what Part O does not cover. Typical domestic extensions and refurbishments to existing homes are generally out of scope — Part O is aimed at new build. Hotels are not treated as residential for these purposes. If you are doing a loft conversion or a rear extension, you are not normally building to Part O. If you are putting up a new dwelling, you are.

One practical note: Part O is an England regulation. Wales, Scotland and Northern Ireland have their own building standards and overheating provisions, so if you work across borders do not assume the same document applies.

The Two Routes to Compliance

Approved Document O gives you a choice of how to demonstrate compliance. You can use the simplified method or the dynamic thermal modelling method. Pick the wrong one early and you can end up redesigning glazing late in the day.

1. The Simplified Method

The simplified method is a set of prescriptive rules. It is the cheaper, faster route because it needs no specialist software — but it is also the more restrictive, because it has to be conservative enough to work across a wide range of homes.

It works on two fronts. First, it caps the glazing area by orientation: there are maximum glazed areas as a proportion of floor area, and the limits are tighter for windows facing the hot side of the building (broadly the east-through-west arc that catches morning to evening sun) than for north-facing glazing. Second, it sets minimum free opening areas for ventilation so occupants can purge heat — again expressed as a proportion of floor area.

The document also separates locations into normal-risk and high-risk categories. High-risk locations — for example densely built parts of central London and other heavily urban areas — face stricter glazing limits and higher ventilation requirements, because external temperatures are higher and opening windows is more constrained by noise and security. If your site falls into a high-risk location, the simplified route gets harder and dynamic modelling often becomes the more economical choice.

2. Dynamic Thermal Modelling (TM59)

The second route is a full dynamic thermal model of the building, assessed against CIBSE's TM59 methodology for domestic overheating. A modeller simulates the building hour by hour across a design summer year and checks whether the internal temperatures stay within the TM59 criteria — including specific limits on overheating in bedrooms at night.

This route is more expensive and needs a qualified assessor, but it is far more flexible. Because it models the actual building rather than applying blanket caps, it lets you justify larger areas of glazing, unusual layouts or specific shading strategies that the simplified method would simply reject. On apartment schemes and architecturally ambitious houses, dynamic modelling is often the only way to keep the design the architect and planners signed off.

The Key Design Levers

Whichever route you take, compliance comes down to a handful of physical decisions. Understanding these helps you steer a job before it gets to building control rather than after.

• Window size and orientation. The single biggest driver of solar gain. Large areas of glazing facing the sun — particularly south and west — are the most common cause of failure. Reducing the glazed proportion, or moving glazing to cooler elevations, is the most direct fix.
• Shading. External shading is far more effective than internal blinds at keeping heat out. Overhangs, brise-soleil, recessed reveals, external shutters and even balconies on the floor above all cut solar gain. Fixed shading is reliable; it cannot be removed by occupants.
• Openable windows. Purging heat depends on occupants being able to open windows — but the document recognises this conflicts with noise, security and air pollution. On a busy road, a window that cannot realistically be left open at night does not count the same way it would on a quiet street. This is exactly where high-risk locations bite.
• Cross-ventilation. Openings on opposite sides of a dwelling let air flow through and remove heat far faster than single-sided ventilation. Dual-aspect flats are much easier to make compliant than single-aspect flats, where one external wall does all the work.
• Mechanical cooling. Treated as a last resort. The hierarchy is: limit gains first, remove heat passively second, and only then add mechanical cooling. Relying on air conditioning to pass Part O is discouraged and pushes up both capital cost and the building's energy use.

The Tension With Part L and Part F

Part O does not exist in isolation, and the regulations can pull against each other. This is where coordination on a build matters most.

Part L (energy efficiency) wants you to capture useful solar gain in winter and keep the fabric airtight and well insulated. Part O wants you to reject solar gain in summer and let heat escape. South-facing glazing is great for Part L and a liability for Part O. The resolution is usually shading that blocks high summer sun while admitting low winter sun, plus careful sizing of glazing — but it means the two compliance exercises have to be done together, not in sequence.

Part F (ventilation) overlaps directly with Part O's requirement to remove excess heat, but the two are not the same. Part F is about indoor air quality and background ventilation; Part O is about purging heat. A mechanical ventilation system designed purely for Part F air quality will not on its own satisfy Part O's need to dump heat on a hot day. The ventilation strategy has to be designed to satisfy both at once, and the assessor needs to see how openings, trickle vents and any mechanical systems work together.

What Gets Checked and Documented?

Building control will want evidence that the chosen route has been followed. The exact paperwork depends on which method you used:

• Simplified method: glazing areas by orientation set against the floor area, the free opening areas provided for ventilation, the cross-ventilation strategy, and the location risk category. A compliance checklist with this design data is normally expected.
• Dynamic modelling: the full TM59 assessment report from the assessor, showing the modelling assumptions and that the building meets the temperature criteria — including the night-time bedroom checks.
• Shading and openings: details of any fixed external shading and the type of windows specified, since restricted opening (for safety) reduces the effective free area.
• As-built consistency: what is fitted on site must match what was assessed. Swapping a specified shading device or changing a window restrictor can quietly invalidate the assessment.

Keep this documentation with the rest of the job's compliance pack. If a window schedule or shading detail changes during the build, flag it back to whoever did the Part O assessment before it goes in.

Common Pitfalls on South and West-Facing Glazing

Almost every Part O headache traces back to glazing on the hot elevations. The classic failures are:

• Big south-facing windows added for the view or the brochure. They look great in a marketing render and fail Part O. West-facing glazing is often worse than south because the afternoon sun is low and strikes the glass directly, peaking when the building has already warmed up.
• Relying on internal blinds. Blinds and curtains do little for overheating because the heat is already through the glass. Only external shading is properly effective, and the simplified method does not give you credit for internal measures.
• Single-aspect flats on busy roads. No cross-ventilation, and windows that cannot be opened at night because of noise or security. This combination is the hardest of all to make compliant and often forces a redesign or a move to dynamic modelling.
• Late value-engineering. A window restrictor added for child safety, or a balcony deleted to save cost, can remove the very shading or free opening area the assessment relied on. Changes made after the Part O sign-off are a frequent cause of failures at completion.

The practical takeaway: get the overheating assessment done early, alongside Part L, and treat the glazing schedule, shading details and window specification as fixed once they are signed off. Changing them mid-build is where the money and the delays come from.

Frequently Asked Questions

Does Part O apply to my extension?

In most cases, no. Part O is aimed at new residential buildings. Typical domestic extensions and refurbishments to existing homes are generally outside its scope. If you are creating a new dwelling, it applies.

Which compliance route is cheaper?

The simplified method is cheaper up front because it needs no specialist software. But it is restrictive, so on glazing-heavy designs or high-risk locations the cost of cutting window areas can outweigh the price of dynamic modelling — which may let you keep the design as drawn.

Can I just fit air conditioning to pass?

Mechanical cooling is treated as a last resort, not a default solution. You are expected to limit solar gains and remove heat passively first. Leaning on air conditioning pushes up capital cost and energy use and works against Part L, so building control will expect to see the passive measures addressed first.

What is TM59?

TM59 is the CIBSE methodology used to assess domestic overheating under the dynamic modelling route. It simulates the building across a design summer and checks internal temperatures against set criteria, including specific limits for overheating in bedrooms at night.

Does Part O apply outside England?

Approved Document O is an England regulation. Wales, Scotland and Northern Ireland have their own building standards covering overheating, so check the relevant national guidance if you build across borders.