Last updated: July 2026

If your Part O overheating assessment has come back with a fail, the design isn't necessarily the problem. Part O regularly conflicts with three other parts of the Building Regulations, Part Q (security), Part B (fire), and Part L/M&E strategy, and a fail on paper often has nothing to do with whether the building actually overheats.

Below are the five most common causes of Part O failure we're seeing on projects right now, plus the regulatory conflicts behind each one.

1. You've used the Simplified Method when the project needed dynamic modelling

The Simplified Method in Part O is deliberately conservative: it ignores thermal mass, ignores internal shading, and doesn't account for passive design measures at all. As a result, developments that fail the Simplified Method frequently pass full TM59 dynamic modelling with no design changes at all, same building, same windows, just a more accurate calculation method.

What to do: If you believe the design is genuinely fine but it's failing on paper, don't redesign yet, get it modelled properly under TM59 first.

2. Part O and Part Q are pulling in opposite directions on ventilation

Part O typically relies on bedroom windows being open at night for ventilation. But under Part Q (security), any window within 2m of ground level has to be modelled as closed from 11pm to 7am, regardless of whether it physically opens, because it's classed as easily accessible.

What to do: If a ground-floor design depends on a window opening at night to pass Part O, check Part Q before you rely on it. Building control will.

3. One window, two regulations, barely any tolerance

Where a window doubles as a fire escape, Part B sets a maximum sill height of 1100mm so occupants can climb out. Where that same window is part of the overheating strategy, Part O sets a minimum sill/guarding height of 1100mm, because the window is expected to stay open for extended periods.

Government guidance allows a build tolerance of +0/–100mm, meaning there's effectively no margin between the two requirements.

4. The ventilation strategy assumes behaviour that won't happen in practice

A Part O Overheating Assessment isn't just asking whether a window can open, it's asking whether someone will actually open it at 2am with traffic noise outside. Where nighttime noise levels are high, residents tend to keep windows shut, which can mean the ventilation strategy doesn't legally count.

The same conflict shows up with fire doors: TM59 modelling often assumes internal doors stay open to purge heat through a dwelling, but fire-rated doors are required to stay shut. Mechanical ventilation isn't an automatic fix either, MVHR without adequate passive design still fails TM59 regularly.

5. The overheating assessment was left too late

Overheating needs to be assessed at concept design stage, while layouts and window sizes can still be changed cheaply. Leaving it until technical design means a fail can force a window redesign or mechanical cooling, which then creates a second compliance issue under Part L.

What to do: Catch it early. It's a five-minute conversation at concept stage instead of a five-figure redesign later.

Where Part O Overheating Assessments stand in 2026

In the Future Homes Standard consultation response published in March 2026, government confirmed that Part O will get its own full technical review, specifically because of the conflicts outlined above. No date has been published for revised guidance, and the current 2021 rules remain fully in force, everything above reflects what building control is checking against today.

FAQs

Can a Part O fail on the Simplified Method be overturned without changing the design? Often yes. The Simplified Method is conservative by design and ignores thermal mass, shading, and passive measures. Full TM59 dynamic modelling frequently passes the same design that failed the Simplified Method.

Do ground-floor bedroom windows count for Part O ventilation if they're covered by Part Q? Not overnight. Any window within 2m of ground level must be modelled as closed from 11pm–7am under Part Q, even if the physical window can open, so it can't be relied on for Part O nighttime ventilation during those hours.

What's the tolerance between Part B and Part O sill height requirements? Effectively none. Part B sets a maximum of 1100mm for fire escape windows; Part O sets a minimum of 1100mm for overheating ventilation windows where guarding is required. Build tolerance is +0/–100mm.

When should a Part O overheating assessment be carried out? At concept design stage, before window sizes and layouts are fixed. Leaving it until technical design risks a costly redesign or reliance on mechanical cooling, which can trigger separate Part L issues.

Are Part O Overheating Assessments due to change? The Future Homes Standard consultation response (March 2026) confirmed Part O will undergo a full technical review. No revised guidance date has been set, so the 2021 rules currently apply in full.

Stay ahead of Building Regs changes

Part O is under formal review, and the Future Homes Standard is reshaping compliance requirements across the board. For practical breakdowns of what's changing and what building control is actually checking for, follow the Energy Digest YouTube channel — we publish regular explainers on Part O, TM59, Energy Statements, and the regulatory conflicts that catch projects out.