GOV.UK’s guidance on Approved Document O states that it applies to new residential buildings, and does not extend that scope to extensions or conservatories added to an existing dwelling, meaning the overheating limits and assessment methods in Part O are not the regulatory route that governs a conservatory’s design against overheating.

What Part O actually is

Part O of Schedule 1 to the Building Regulations sets requirements aimed at limiting unwanted solar gain and providing ways to remove excess heat from residential spaces, addressing the risk that increasingly airtight, well-insulated new homes can overheat in summer. Approved Document O, the guidance supporting these requirements, took effect from 15 June 2022, giving designers a route to demonstrate compliance through either a simplified method or dynamic thermal modelling.

Who it applies to

GOV.UK’s own description of the scope is direct: Approved Document O applies to new residential buildings. This is a narrower scope than the Building Regulations as a whole, which cover many kinds of work to existing buildings as well as new ones, and it means Part O’s specific overheating assessment requirements are targeted at new dwellings being designed and built from scratch.

Why a conservatory is different

A conservatory added to an existing home is an extension, not a new residential building in its own right, and GOV.UK’s guidance does not extend Part O’s scope to this kind of work. This means the specific overheating limits, assessment methods and compliance routes set out in Approved Document O are not the regulatory mechanism that applies when a conservatory is designed, even though a conservatory is, in ordinary terms, exactly the kind of heavily glazed space most likely to overheat.

What still governs heat performance

This does not mean a conservatory’s thermal performance is unregulated. Where a conservatory falls outside the Schedule 2 exemption covered elsewhere on this site, and the full Building Regulations apply, requirements for thermal separation, glazing performance and energy efficiency covered under other parts of the Regulations still apply to the extension. Where the conservatory is within the exemption, its thermal performance is instead a matter of design choice and product specification, covered in more practical terms elsewhere on this site, such as glazing U-values and shading, rather than a Part O compliance exercise.

Why the confusion happens

Part O was introduced specifically because of concern about overheating risk in modern housing, and a conservatory is a very visible, everyday example of a space prone to overheating, which is likely why homeowners and even some installers assume the new regulation must apply to it. The scope GOV.UK sets out, new residential buildings only, is narrower than that assumption, and confirming this distinction avoids treating a Part O compliance exercise, designed for new-build design teams, as something a conservatory project needs to go through.

What this means in practice

For a homeowner adding a conservatory, the practical route to avoiding an overheated space remains the design and product choices covered elsewhere on this site: glazing specification, shading, ventilation and orientation, rather than a Part O assessment. For a homeowner buying a genuinely new-build home that includes a conservatory or similarly glazed space as part of the original design, Part O will have applied to the whole new dwelling at the design stage, including that space, because the building as a whole was new construction rather than an extension to an existing home.

Why installers shouldn’t cite Part O as a requirement

Because Part O is relatively recent and specifically framed around a genuine overheating problem in new housing, a homeowner may come across it mentioned in connection with a conservatory quote or specification. Given GOV.UK’s stated scope, an installer citing Part O compliance as something legally required for an extension to an existing home is describing a stricter position than the regulation itself sets out; that does not mean good glazing and shading specification are unimportant, only that they are a design and performance choice for a conservatory, not a Part O legal compliance exercise in the way they would be for a new dwelling.

FAQs

Does Approved Document O apply to a conservatory added to an existing home?

No. GOV.UK’s guidance says Approved Document O applies to new residential buildings, not to extensions such as a conservatory.

When did Approved Document O come into force?

15 June 2022, for new residential buildings in England.

Is a conservatory’s overheating risk unregulated because Part O doesn’t apply?

Not entirely; other Building Regulations requirements can still apply where the conservatory isn’t exempt, and overheating is otherwise addressed through design choices like glazing, shading and ventilation.

Does Part O apply to a new-build home that includes a conservatory-style space in the original design?

Yes, in that case the whole dwelling is new residential construction, which is within Part O’s stated scope.

Bottom line

Approved Document O, the Building Regulations guidance on limiting overheating that took effect from 15 June 2022, applies to new residential buildings only, and GOV.UK’s guidance does not extend this to extensions such as a conservatory added to an existing home. A conservatory’s overheating risk is instead managed through design choices and, where relevant, other Building Regulations requirements, not a Part O compliance exercise. This is general information, not advice on a specific project; a designer or building control body can confirm which regulations apply to a particular scheme.

Sources

This guide draws on the following primary sources, current as of 22 September 2026:

• GOV.UK, “Overheating: Approved Document O”

Related guides

Compare Conservatories is an independent guide. We may earn a fee from some links; this never affects what we write. This article is general information, not legal or planning advice on any specific property — always confirm requirements directly with your local planning authority.