Trades master copy
Last reviewed: March 2026
What Part O is
Part O is the overheating bit of Building Regulations. It only applies to new residential buildings (new houses, flats, care homes, student blocks etc.), not extensions or conservatories added later.
The aim is to stop new homes turning into ovens in summer by:
- Limiting unwanted solar gains.
- Providing a decent way to dump heat (mostly through opening windows/vents).
This guide is a summary to make Part O easier to use on site. It does NOT replace Approved Document O: Overheating (current edition).
You must read and follow the full Approved Document O and the overheating assessment (simplified tool or dynamic modelling) for your project.
This guide is written for England. Wales, Scotland and Northern Ireland have their own versions of building regulations - the principles are similar but the documents and approval routes differ, so check local requirements if you're working outside England.
Where it applies on your jobs
Part O bites when:
- You build new dwellings (houses or flats).
- You build new residential buildings like student housing, care homes, etc.
- Those buildings have decent amounts of glazing, especially on south/west facades, and limited cross-ventilation.
It does not normally apply to:
- Extensions to existing houses.
- Conservatories added after the house was built (but still watch Part L/F and common sense on overheating).
Key "trigger points" - what matters on site
Limiting solar gains (glazing, orientation, shading)
- The more glass you have on south and west faces, the harder Part O is to pass - especially for single-aspect flats without cross ventilation.
- Under the Simplified Method in AD O, there are hard limits on:
- Total glazing area as a % of floor area.
- Glazing in the most glazed room as a % of that room's floor area, with tighter limits for south/west orientations.
- External shading (overhangs, shutters, brise-soleil) or high-performance glass can be used to keep those solar gains down.
Removing excess heat (window openings / free area)
- Part O sets minimum "free area" of openable windows/vents - basically how much opening you actually get when the sash is open, as a % of floor or glazing area.
- For typical moderate-risk locations (most of England outside hot bits of London), the simplified method expects:
- Total free area across the dwelling ≈ at least 9% of total floor area or 55% of total glazing area (whichever is bigger).
- In bedrooms, free area ≈ at least 4% of that bedroom's floor area.
- Single-aspect and corner flats with limited openings are much harder to pass on the simplified method and often need full dynamic modelling (TM59).
High-risk locations
- Urban parts of London and some dense urban areas are defined as high-risk for overheating - Part O is tighter there (more shading, less glass, more free area).
- The Approved Document includes postcode-based lists and maps; the overheating assessor will factor this in.
Mechanical cooling (air-con)
- Part O wants you to use passive measures first - shading, sensible glazing, ventilation - and only add mechanical cooling if you've genuinely run out of passive options.
- If you do install mechanical cooling, you still have to show you've done everything reasonably practicable on glazing/shading/ventilation.
Quick reference table - common jobs
New south-facing house with big bifolds
- Total and room-by-room glazing must stay within the simplified method limits, or you shift to dynamic modelling.
- You may need external shading (overhangs, shutters) and decent-size opening windows to hit both solar-gain and free-area targets.
New single-aspect flat in a block
- Single-aspect dwellings are tough under the simplified method, especially if heavily glazed.
- You might need reduced glazing, better shading, or full dynamic modelling (TM59) to prove compliance.
- Window free area still has to meet the minimum percentages.
New block of flats in London (high-risk location)
- Stricter glazing and ventilation limits under the simplified method; external shading is often needed.
- You'll frequently be into dynamic modelling rather than relying on the simple tables.
- Coordination with facade, balcony and window design is critical.
House with MVHR and talk of adding AC later
- You still have to show overheating is controlled mainly by passive means (glazing, shading, window opening) even if mechanical cooling is proposed.
- Part O compliance is separate from Part L/SAP, but the design decisions interact (glazing, orientation, thermal mass).
Routes to compliance for trades
You won't be doing the calc yourself, but you can either make it easy or make it a nightmare.
Two compliance routes - know which one you're on
Simplified method (AD O tables):
- Uses glazing limits by orientation and minimum opening "free area" targets.
- Good for simple, modestly glazed homes in moderate-risk areas.
Dynamic thermal modelling (e.g. CIBSE TM59):
- More flexible, used when the simplified method is too restrictive or fails.
- Essential for many single-aspect/high-risk or heavily glazed designs.
What you need to do on site
Build glazing and openings exactly to the overheating design:
- Don't upsize bifolds or add extra rooflights because "it will look great" without sending it back to the Part O assessor.
- Don't downsize opening sashes or swap to restricted hinges that slash free area.
Install any specified shading (overhangs, blinds, brise-soleil) and light-coloured/high-performance finishes where called for.
Coordinate with Parts L and F
Glazing and opening decisions affect:
- Part L (energy - you still need enough solar gain in winter).
- Part F (ventilation - openings and trickle vents).
Don't let one Part be "fixed" on site at the expense of the others; keep your energy/overheating person in the loop.
Who is responsible for what
On a typical new-build project:
- The architect/energy/overheating assessor is responsible for choosing the Part O route (simplified vs modelling), setting glazing, shading and opening sizes, and producing the overheating assessment.
- The builder/main contractor is responsible for actually building that spec: correct glass areas by orientation, correct opening types/sizes, and installing any shading and ventilation kit.
- Window/facade suppliers are responsible for supplying frames and hardware that deliver the designed free areas and performance - not quietly swapping to smaller openers or more restrictive hinges.
- The client/developer carries the risk if they push for "more glass" or last-minute design changes without re-checking Part O.
Blunt version:
If you agree to "make the bifolds wider" or "shrink those openers a bit" on a Part O-tested design without sending it back to the overheating assessor, you're gambling with compliance. Fixing it afterwards often means ripping out expensive glass.
Simple rule to drum into your team
If you're building new dwellings, any change to glazing size, orientation, opening style, or shading is a Part O issue. Don't tweak those on the fly - get the overheating/SAP person to bless it first.
This guide was last reviewed March 2026. SiteKiln does not provide legal, financial or tax advice. All content is for general information purposes only. Always seek professional advice for your specific situation.
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Working in Wales? The building rules are different. See our Working in Wales guides.
Working in Scotland? Building standards work differently. See our Working in Scotland guides.
Working in Northern Ireland? The system uses Technical Booklets. See our Working in Northern Ireland guides.
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