Overheating has been a Building Regulations problem since Approved Document O landed in 2021. You design the home, you run the simplified method or a TM59 dynamic model, you hand the results to Building Control, and planning barely touches it.
The National Planning Policy Framework published on 17 August 2026 changes where that conversation starts.
Overheating was not absent from the previous Framework — the December 2024 version mentioned it in its general climate framing (paragraph 161) and in its plan-making paragraph on long-term risks (162). What it did not have was a decision-making policy telling applicants to design for it. The 2026 rewrite does.
What the new policies actually say
The Framework’s climate chapter contains three coded policies, CC1 to CC3. Two of them mention overheating directly.
CC1 is the plan-making policy. It requires development plans to take a proactive approach to adapting to climate change, “taking into account the implications of extreme weather and long-term climate trends including overheating, wildfires, drought, flood risk, coastal change, water supply, biodiversity and landscapes.”
CC3 is the national decision-making policy on adaptation, and it is the one that will show up on your desk. It requires development proposals to take account of the current and potential impacts of climate change over the lifetime of the scheme and, where relevant, to:
“Use design approaches which minimise risks from overheating in accordance with policy DP3(2)(b), and include green infrastructure and suitable tree planting in accordance with policies DP3(2)(c) and N3”
Two things follow from that wording. First, overheating sits alongside flood risk, coastal change and SuDS in the same list — the company it keeps tells you how seriously it is meant to be taken. Second, it routes through DP3, the design policy in the well-designed places chapter, which means overheating is being framed as a design-quality issue rather than a bolt-on calculation.
Why this matters now
Because the Building Regulations side has not caught up.
Approved Document O (2021) remains in force and still references the 2017 edition of CIBSE TM59. The government confirmed in its Future Homes and Buildings Standards consultation response on 24 March 2026 that Part O would not be amended alongside Parts L and F, and would instead get its own standalone technical review — covering, among other things, adoption of the updated TM59 methodology, fixes to the simplified method, noise and security guidance, extension to material change of use, and updated weather files.
Meanwhile CIBSE published TM59:2026 in July 2026, a substantial rewrite of the methodology that Part O does not yet point to.
So the sequence is now: planning policy names overheating in August 2026; the industry methodology was rewritten in July 2026; and the Approved Document that governs compliance still references a 2017 document. That gap is where arguments happen.
What it means in practice
Nothing here creates a new numerical standard. CC3 does not set a temperature threshold, and it does not require a TM59 model. What it does is give a local planning authority a clear, named national policy hook to ask how a scheme deals with overheating — at application stage, before Building Control ever sees it.
Expect that to surface as:
- Validation and consultee questions on larger or higher-risk schemes — single-aspect flats, high glazing ratios, urban sites where opening windows is constrained by noise or air quality.
- Design-and-access or sustainability statements being expected to say something specific about overheating strategy, rather than deferring it to “Part O at technical design”.
- Planning conditions requiring an overheating assessment or a mitigation strategy to be submitted and approved. Once that condition exists, the assessment method and the acceptance criteria become negotiable in a way they are not under Part O.
- Tension between policies. CC3’s answer to overheating is passive design, shading and green infrastructure — which can pull against glazing, daylight, density and elevational treatment expectations elsewhere in the Framework. Conflicts between Part O and other parts of the Building Regulations are one of the workstreams in the government’s own Part O review, so this is a known problem rather than a theoretical one. The NPPF does not resolve it for you.
What to do about it
If you are working on schemes going into planning now:
- Do the overheating thinking at concept stage, not at Building Regs stage. Orientation, glazing ratio, aspect and shading are cheap to change on a layout drawing and expensive to change once the planning permission fixes the elevations.
- Be explicit in the planning submission. A short, honest paragraph on the overheating strategy — passive measures first, what you have assumed about openable windows, what you would do if the dynamic model came back tight — is much easier than answering it cold after a consultee objection.
- Watch which TM59 you are being asked for. Approved Document O still means the 2017 edition. A planning condition drafted loosely as “in accordance with CIBSE TM59” now arguably means the 2026 edition, which is a different test with different criteria. Get that pinned down in the wording rather than discovering it at discharge.
- Do not assume Part O compliance settles the planning question. It usually will. But CC3 is drafted around risk and design approach, not around a compliance certificate, so a scheme that scrapes through the simplified method may still attract questions.
The bigger picture
This is a familiar pattern. Water efficiency and sustainable drainage both moved from technical guidance into named planning policy before the Building Regulations changed, and both ended up being fought over at application stage. Overheating now looks set to follow the same route — with the added complication that the review of Part O is still open, so the technical standard the planning system is implicitly reaching for has not yet been written.
If you have a scheme where overheating is likely to be the awkward bit — flats, constrained urban sites, high glazing, or anywhere a planning officer has already asked the question — get in touch and we’ll tell you how we’d approach the assessment and what evidence is worth putting in front of the planners.
Sources: National Planning Policy Framework, August 2026 (PDF), GOV.UK — chapter 5, policies CC1–CC3 · The Future Homes and Buildings Standards — written statement, UK Parliament, 24 March 2026 · Future Homes and Buildings Standards consultation response (PDF), GOV.UK · Government announces full review of Overheating Regulations, Elmhurst Energy, 9 April 2026