Articles

Why Your Part O Overheating Assessment Is Failing

Read time: 5 minutes
June 24, 2026

If your Part O overheating assessment has failed, it's rarely because the design is wrong. More often, it's because Part O conflicts with itself, and with several other Approved Documents, in ways that aren't obvious until building control flags them.

In this article:

  • Why the Simplified Method fails designs that would pass dynamic modelling
  • How the Part Q security rules cancel out Part O's ventilation strategy on ground floor windows
  • Why the same window can need a sill height of both "no more than 1100mm" and "no less than 1100mm"
  • Why a technically-open window doesn't always count as ventilation
  • When in the design process an overheating assessment should actually happen
  • What's changing, as Part O is currently under government review

What is Part O, and why does it conflict with other regulations?

Part O is the Approved Document covering overheating in new residential buildings in England, in force since 15 June 2022. It was the first standalone overheating regulation in English Building Regs history, before that, overheating was a minor sub-section of Part L's energy calculations.

There are two ways to demonstrate compliance:

  • The Simplified Method — fixed limits on glazing area, ventilation openings, and shading.
  • Dynamic Thermal Modelling (CIBSE TM59) — a full simulation of how the building performs under a defined weather file and occupancy pattern.

The conflicts below exist because Part O was added on top of an existing rulebook, Parts B, K, Q, and L, without those documents being rewritten to match it. Part O is currently under formal government review for exactly this reason: in its Future Homes Standard consultation response, published 24 March 2026, government confirmed Part O will get its own technical review. There's no published date yet for revised guidance, and the 2021-edition rules remain fully in force today.

Reason 1: You used the Simplified Method when the design needed dynamic modelling

Short answer: the Simplified Method is deliberately conservative, and it fails designs that would pass proper modelling.

The Simplified Method works from fixed glazing percentages, fixed ventilation openings, and binary shading rules. It doesn't account for:

  • Thermal mass
  • Internal shading
  • Passive design strategies
  • The real-world benefit of triple glazing

These are exactly the factors government's own industry feedback flagged as missing. The practical result: developments that fail the Simplified Method frequently pass full TM59 dynamic modelling with zero design changes. Same building, same windows — just a calculation method that actually reflects how the building performs.

Simplified Method versus Dynamic Thermal Modelling for Part O complianceComparison diagram of the two Part O compliance routes. The Simplified Method is quick and cheap but conservative, ignoring thermal mass, internal shading and passive design, so it can fail designs that are actually fine. Dynamic Thermal Modelling using CIBSE TM59 is more accurate and accounts for those factors, but takes longer and costs more to run.Two routes to Part O complianceSimplified Method+ Quick to run+ Low cost, no software needed+ Simple pass/fail check− Ignores thermal mass− Ignores internal shading− Can fail designs that wouldpass dynamic modellingDynamic Modelling (TM59)+ Accounts for thermal mass+ Accounts for shading,passive design, glazing type+ Reflects real performance− Takes longer to run− Higher cost− Needs specialist softwareorSame building, different outcomeA design that fails the Simplified Methodfrequently passes TM59 dynamic modelling,with zero changes to the building itselfSource: Government industry feedback on Approved Document O

What to do about it: if a design has failed the Simplified Method but the fundamentals look sound, decent orientation, sensible glazing, workable ventilation, get it checked through dynamic modelling before committing to a redesign. The fix may be in how it's assessed, not how it's built.

Reason 2: Does Part Q cancel out your Part O ventilation strategy?

Short answer: often, yes, if the window is on the ground floor or otherwise classed as "easily accessible."

Part O's overheating strategy usually relies on bedroom windows opening at night for ventilation. Part Q and wider Secure by Design guidance take the opposite view on accessible windows: anything within roughly two metres of ground level is expected to be secured, which in practice limits how far it can open.

Government's own AD-O FAQ confirms the resulting rule directly: easily accessible windows should be modelled as closed between 11pm and 7am, even if they're physically able to open. If a ground-floor unit's overheating strategy depends on that window opening overnight, the assessment has to assume it doesn't.

Part O versus Part Q conflict on ground floor windowsDiagram showing how Part O requires a ground floor bedroom window to be open at night for ventilation, while Part Q requires the same window to be modelled as closed at night for security, because it is classed as easily accessible.Ground floor bedroom window, 11pm–7amPart OOverheatingWindow must be openfor night-time purgeventilationPart QSecurityEasily accessible windowmust be modelled asclosed overnightvsSame window, contradictory ruleIf within ~2m of ground level, the window istreated as closed 11pm–7am for security —even though it can physically openSource: GOV.UK Approved Document O FAQ

Common ways around this:

  • Secure grilles or lockable restrictors that still permit night-time airflow
  • High-level fanlights, positioned above the two-metre accessibility threshold
  • Insulated secure louvres
  • Mechanical purge ventilation, modelled through TM59 instead of relying on an openable window

What to do about it: check the Part Q classification of every window in your overheating strategy before assuming it can be modelled as open overnight.

Reason 3: Can the same window satisfy Part B and Part O sill height rules?

Short answer: only within a 100mm margin, and most designs don't check both documents at the same time.

This contradiction is confirmed in both GOV.UK's official AD-O FAQ and the RICS Built Environment Journal:

  • Part B sets a maximum sill height of 1100mm for fire escape windows, so someone can climb out in an emergency.
  • Part O sets a minimum sill or guarding height of 1100mm for any window relied on in an overheating strategy, since it's expected to stay open for long periods.

Where a window does both jobs, common in single-aspect flats and conversions with limited window choice, it needs a sill height of no more than 1100mm and no less than 1100mm at the same time. Government's own guidance accepts a build tolerance of just +0mm / −100mm. That's the entire margin available.

Part O versus Part B conflict on escape window sill heightsDiagram showing how Part B requires a fire escape window to have a maximum sill height of 1100mm so a person can climb out, while Part O requires the same window to have a minimum sill or guarding height of 1100mm for fall safety, leaving only a 100mm tolerance to satisfy both.Same escape window, sill heightPart BFire escapeSill must be no higherthan 1100mm, so a personcan climb outPart OOverheatingSill or guarding must beat least 1100mm, for fallsafety while openvsMaximum and minimum, same numberA sill must be no more than 1100mm and noless than 1100mm at once. Official buildtolerance: +0mm / −100mm, the entire marginSource: GOV.UK Approved Document O FAQ; RICS Built Environment Journal

What to do about it: check sill heights for any escape window that also forms part of the overheating strategy against both documents simultaneously at design stage, not one document first and the other assumed compliant afterward.

Reason 4: Why doesn't an openable window always count as ventilation?

Short answer: because Part O assumes occupants behave realistically, not theoretically.

A ventilation strategy that looks fine on a drawing can fail because of how it would actually be used:

  • Noise. If night-time external noise exceeds roughly 40dB LAeq over 8 hours (11pm–7am), or has frequent loud peaks, residents are assumed to keep windows shut. Near a busy road or railway? That window may not count as valid mitigation, regardless of what the model shows.
  • Fire doors. TM59 modelling often assumes internal doors stay open to purge heat through the home. But fire-rated doors are required to stay shut under Part B, a direct, currently unresolved conflict between the two documents.
  • MVHR isn't a standalone fix. Mechanical ventilation without passive design alongside it, sensible glazing, shading, some openable area, still fails TM59 regularly.

The underlying question to ask: will a real occupant actually open this window, at 2am, with traffic noise outside and a fire door in the way? If the honest answer is no, the model needs to reflect that before submission.

Reason 5: When should a Part O assessment actually happen?

Short answer: at RIBA Stage 2–3, concept and developed design, while layouts and window sizes can still change cheaply.

Leave it until Stage 4 (technical design) or later, and a failure means either redesigning windows and layouts, or bolting on mechanical cooling afterward.

The second option has a knock-on effect that's easy to miss: cooling added to fix an overheating failure increases energy demand, which then has to be reflected in the Part L energy calculation, potentially creating a second compliance problem. This "double penalty" was significant enough that government addressed it directly in the March 2026 Part L update: where air conditioning or low-g glazing is added specifically to meet Part O, the Part L calculation now accounts for it, so developers aren't penalised twice.

What to do about it: an overheating screening review at concept stage is a far cheaper conversation than a Part L resubmission after a late-stage Part O failure.

What does this mean for your project?

None of these five issues come from poor design. They come from several Approved Documents, O, B, K, Q, and L, having been written largely independently of each other, with the job of reconciling them left to the design team.

Until Part O's review produces updated guidance, the practical approach is to check any overheating strategy against Parts B, K, Q, and L at the same time as Part O, not as a separate, final step.

If your project is approaching concept or developed design and the overheating strategy hasn't been tested through dynamic modelling yet, that's the cheapest point to find out whether it works.

This article reflects Approved Document O guidance, the official AD-O FAQs published by GOV.UK, and CIBSE TM59 documentation current as of June 2026. Part O is a complex and currently evolving area of the Building Regulations, subject to a confirmed government review. Building control bodies retain final discretion on individual cases, and project-specific advice should always be confirmed directly with building control or a qualified assessor.

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