Does Part O apply to my existing house?

No. It applies to new residential buildings in England. Work on an existing home is not covered by it, which is why the great majority of British housing has no overheating standard attached to it at all and why retrofit shading and ventilation are still a personal decision rather than a regulated one.

Does Part O apply in Scotland, Wales or Northern Ireland?

No. Building regulations are devolved and Part O is an England measure. Scotland, Wales and Northern Ireland each set their own standards through their own systems, so anything written about Part O should not be assumed to describe the position anywhere outside England.

Does a Part O compliant home stay cool in a heatwave?

It is not a guarantee. Part O sets a design standard, assessed at the point the building is approved, using assumptions about how it will be occupied and ventilated. A flat that meets it can still get uncomfortable if the windows cannot safely be opened at night, which is one of the specific things the guidance asks designers to think about.

How can I tell whether a new flat was designed for overheating?

Ask which route the design used, whether the assessment assumed windows would be open at night, and what the assumption was about external noise and security. A developer who can answer those three questions has an assessment. One who cannot is quoting a brochure.

The overheating rule that applies to new homes, and the ones it leaves out

Q: What is Part O of the building regulations?

Part O is the overheating requirement in Schedule 1 to the Building Regulations in England. It asks for reasonable provision to limit unwanted solar gain in summer and to provide an adequate means of removing excess heat from the indoor environment. Approved Document O is the technical guidance that sits underneath it.

Britain builds houses for a climate it no longer has. That was true for decades and it is the reason most of the housing stock behaves the way it does in July: heavy, south facing glazing, tight construction, and no plan at all for getting heat back out.

Since 2022 there has been a rule about it, for new homes in England. It is called Part O, almost nobody buying a house has heard of it, and understanding what it does and does not cover tells you a great deal about which homes will be comfortable in fifteen years.

What the rule actually says

Part O sits in Schedule 1 to the Building Regulations in England, alongside the parts covering structure, fire and ventilation. The requirement itself is short. Reasonable provision must be made to limit unwanted solar gain in summer, and to provide an adequate means of removing excess heat from the indoor environment.

Two halves, and they matter in that order. Keeping heat out is the first job; getting it out again is the second. That is the same order this site recommends for an existing home, and it is why external shutters and shading does more than any amount of ventilation once the sun is already in the room.

The technical guidance is in Approved Document O, published in December 2021 and applying to building work from the following summer.

What it does not cover

This is the part worth knowing, because the gaps are wider than the rule.

Existing homes are outside it entirely. Part O applies to new residential buildings. The house you already live in has no overheating standard attached to it, has never had one, and will not acquire one by being extended or refurbished. The overwhelming majority of the homes that will be standing in Britain in twenty years are already standing today, so the rule reaches a small slice of the problem.

It is England only. Building regulations are devolved. Scotland, Wales and Northern Ireland set their own standards through their own systems, and nothing written about Part O can be assumed to apply outside England. If you are reading this in Cardiff or Glasgow, check your own regime rather than this one.

It is a design standard, not a performance guarantee. Compliance is assessed before anyone lives in the building, using assumptions about how it will be used. A home can meet the standard on paper and still be uncomfortable if those assumptions do not survive contact with the occupant.

The night ventilation problem

The single most useful thing in the whole subject is a question the guidance forces designers to confront: can the occupant actually open the windows at night?

British homes shed heat overnight or they do not shed it at all. Our piece on why British bedrooms do not cool down covers the physics. The trouble is that a design which relies on open windows has to be honest about three things:

Noise. A bedroom window on a main road cannot be left open at night by anyone who intends to sleep.
Security. A ground floor or accessible window that must stay open is a security decision the resident will quietly reverse.
Pollution and insects. Both are ordinary reasons a window gets closed in practice.

A design that assumes open windows without addressing those is a design that will fail in occupation while passing on paper. When a developer tells you a flat meets the overheating standard, the follow-up question is what the assessment assumed about the windows.

What to ask before you commit

Three questions, and they take a minute.

Which route was the overheating assessment done by? There is a simplified route based on glazing areas and ventilation openings, and a modelling route where the building is simulated over a summer. A developer who cannot say which was used has not seen the assessment.
Did the assessment assume windows open at night, and where? If yes, go and stand outside at eleven at night and decide whether you would.
What is the plan when it is 32C outside? Not a rhetorical question. The honest answer is usually shading plus night ventilation, and if the honest answer is “you can fit air conditioning later”, that is a planning question as well as a cost one.

That last point is where a lot of buyers get an unwelcome surprise, because fitting an external unit to a flat is not automatically permitted. Our planning permission checker walks through the England rules for a domestic installation, and the crucial catch is covered in cooling only air conditioning and planning: a unit that only cools is treated differently from one that heats as well.

What you can do in a home the rule never reached

Most readers are in an existing house, which is exactly where there is no standard and no assessment, and the honest position is that the whole decision is yours.

The order is the same one Part O implies:

Stop the gain first. External shading beats internal blinds by a wide margin, because a blind on the inside has already let the energy through the glass. West facing rooms are the priority, not south, because late afternoon sun arrives when the house has already been heating all day.
Then move the heat out. Cross ventilation at night, when the outside air is genuinely cooler than the inside air, which is later than most people think.
Only then consider a machine. And size it to the room rather than to the showroom, which is what the room cooling calculator is for. An oversized unit costs more, cycles badly and does not make the room more comfortable.

Why any of this is changing

The reason a rule exists now, and did not in 2010, is that the number of days British homes have to cope with has moved. This site stores the daily record for twenty two towns back to 1950, and the shape of it is visible on every town page: the hot days are not evenly distributed across the record, and the recent decades hold more of them than the early ones.

A house built to 1990 assumptions was built for a different distribution. Part O is an attempt to stop new houses being built for the wrong one, which is worth having, and it does nothing at all for the ones already standing.

Before you rely on any of this

Part O is England only, building regulations change, and the position here describes the guidance as published rather than the detail of any individual scheme. Check the current Approved Document O, linked above, and take proper advice before making a purchase decision on the strength of it.