Summary
- Part O (2021, in force June 2022) applies to new residential buildings only — new dwellings and residential institutions — not to extensions, loft conversions or material change of use. Check applicability first and record the decision.
- Two compliance routes: the simplified method (glazing area limits and free-area ventilation requirements by location and cross-ventilation status) or dynamic thermal modelling to CIBSE TM59.
- The simplified method fails glassy designs fast — in 'high risk' locations (parts of London), maximum glazing areas are tight and openings must deliver defined free areas; check at concept stage before the elevation is sold to the client.
What the Building Safety Act requires for Part O on non-HRB residential jobs
Requirement O1 demands that new residential buildings limit unwanted solar gains in summer and provide adequate means to remove heat from indoor air. It applies to new dwellings and other 'residential' buildings (care homes, student accommodation) but not to extensions to existing homes, and not to buildings created by material change of use. Your first evidence entry is therefore the applicability decision itself.
The simplified method sets maximum glazing areas by orientation, location risk category (high-risk locations are defined postcodes, broadly urban London) and whether the dwelling has cross-ventilation, plus minimum free areas for heat removal. Dynamic modelling to CIBSE TM59 is the alternative for designs that exceed those limits — and it must be commissioned early enough to change the design, not to rubber-stamp it.
Part O also requires that the overheating strategy remain usable in practice: openings relied upon for cooling must be usable at night with reasonable security (ground-floor and accessible windows need justification or alternatives) and noise from outside should not make the strategy unusable — an acoustic constraint on busy roads that surprises many designers. As PD (BSA), record how night-purge security and noise were addressed, because Building Control increasingly asks.
Key design risks and evidence needed (RIBA Stages 2–4)
- Stage 2: applicability check; location risk category; cross-ventilation status; first-pass glazing ratio against the simplified method tables — this is a 30-minute check that can save a facade redesign.
- Stage 3: choose the route (simplified vs TM59); coordinate shading (overhangs, external blinds, low-g glass), free areas and window opening types; address night-time security and noise for openings relied on for purging.
- Stage 4: final simplified-method calculation or TM59 report; window schedule stating free areas and g-values; handover information for the occupant on operating the overheating strategy (required by AD O Section 4).
Standards and references you should be citing
- CIBSE TM59 — design methodology for the assessment of overheating risk in homes (the dynamic modelling route).
- CIBSE TM52 — overheating criteria for non-domestic buildings (referenced for non-dwelling residential spaces).
- BS EN 17037 — daylighting: the counter-pressure to glazing reduction; balancing daylight and overheating is the design task.
- Approved Document O tables for maximum glazing areas and minimum free areas by risk category and cross-ventilation status.
Simplified method vs TM59 modelling
| Aspect | Simplified method | Dynamic modelling (TM59) |
|---|---|---|
| Best for | Conventional glazing ratios, straightforward plans | Glassy designs, single-aspect flats, high-risk locations |
| Cost/time | Hours, in-house | Specialist commission, weeks |
| Flexibility | None — fixed tables | Trades shading, g-values, ventilation, thermal mass |
| Evidence to log | Calculation sheet per dwelling | TM59 report + input assumptions |
Common pitfalls that catch out sole practitioners
- Applying Part O to an extension (it does not apply) or missing it on a new dwelling in a garden plot (it does).
- South-facing glazed gables designed at Stage 2, then 'fixed' at Stage 4 with a TM59 report that fails — model early or glaze modestly.
- Openings counted for free area that cannot realistically be left open at night (ground floor, over a bin store, next to a railway) with no security or acoustic answer recorded.
- Forgetting the AD O requirement to give the occupant written information about how the overheating strategy works.
Frequently asked questions
Does Part O apply to extensions or loft conversions?
No. Part O applies to new residential buildings — new dwellings and residential institutions — not to extensions, loft conversions or material change of use. Record the applicability decision in your requirements tracker so the 'not applicable' conclusion is itself evidenced.
When do I need CIBSE TM59 modelling instead of the simplified method?
Whenever the design exceeds the simplified method's maximum glazing areas or cannot meet its free-area requirements — typically single-aspect flats, high-risk (London) locations, or heavily glazed elevations. Commission it at Stage 2–3 while the facade can still change; a failing TM59 at Stage 4 is a redesign.
Track this natively instead of in Word templates
PD Pathway gives small practices an automated Parts A–T relevant-requirements tracker, stage-by-stage Principal Designer checklists and one-click compliance documents — built strictly for non-HRB projects. First project free, then from £10 per project or £99/yr unlimited.