Why New Homes Fail Part O (and How to Fix It)

Why New Homes Fail Part O (and How to Fix It)

Most Part O failures we see come down to a handful of things: too much glass on one side, windows that don't open far enough, and sites where windows can't be left open at night. All of them are cheaper to fix on the drawings than on site. Here is what goes wrong and what we change first.

1. Too much glazing on the main façade

The simplified method in Approved Document O limits glazing as a share of floor area. The limit depends on which way the most glazed façade faces, and west and south are the hardest. In a moderate-risk location, a home with no cross-ventilation whose most glazed façade faces west can have glazing of only 11% of its floor area. One large set of sliding doors on a west-facing living room is often enough to fail.

Fix: trim window widths, move glass to the north or east side, or split one large opening into smaller ones. Small changes to the window schedule often get a design through.

2. No cross-ventilation

Single-aspect flats get tighter glazing and opening limits. Cross-ventilation only counts when there are openings on opposite façades. A corner flat with windows on two adjacent sides does not count as cross-ventilated.

Fix: where the layout allows, add an opening on the opposite side. If it doesn't, plan for less glazing, or a dynamic model, from the start.

3. Not enough free opening area

Glazing area and opening area are separate checks. Fixed panes, restrictors and the way a window opens all reduce the free area. Each bedroom needs its own free area: at least 13% of the bedroom's floor area in a high-risk location, and 4% elsewhere.

Fix: change the opening type, make more panes openable, or add secure louvred vents or doors with suitable guarding.

4. London's high-risk areas without shading

Much of London is a high-risk location under Part O. There, glazing that faces anywhere from north-east round to north-west via south needs shading. The options are external shutters with ventilation, glass with a g-value of 0.4 or less and light transmittance of at least 0.7, or overhangs on façades facing due south.

Fix: specify solar-control glass early. It is often the simplest route, and it changes the SAP too, so we update both together.

5. Noise, security and fall risk

A window only counts if it can actually be left open. Where noise is an issue, Part O assumes bedroom windows are shut from 11pm to 7am if noise in the bedroom would exceed 40 dB LAeq over that period, or 55 dB LAFmax more than 10 times a night. In ground-floor and easily reached bedrooms, only openings that can be left open securely count, for example with lockable louvred shutters or fixed grilles. Openings with a fall risk need guarding, 1.1m high where the drop is more than 600mm.

Fix: get the noise report and security needs to us early. If bedroom windows have to stay shut at night, the scheme needs a TM59 dynamic model with a ventilation strategy that does not rely on open windows.

6. Communal heating pipework

The simplified method is not suitable for blocks with a communal heating or hot water system that has significant horizontal pipework, usually in the corridors. No change to the glazing will fix that.

Fix: budget for a TM59 model at design stage, not after a failed simplified check.

When the simplified method won't pass

The next step is a TM59 dynamic model. It tests each room hour by hour against summer weather data and gives more freedom on glazing. Part O expects passive measures first: glazing, shading and openings. Mechanical cooling is only accepted where those cannot meet the requirement, and the report has to show they were considered. We look for the affordable route that works for your design.

Check early, not at completion

Once windows are ordered, every fix costs more. We check Part O at design stage, alongside the SAP, so the glazing, g-values and openings match in both. At completion, the person doing the work must give the owner information on the overheating strategy within five days, as part of the Home User Guide.

Part O simplified assessments start from £150+VAT per home and TM59 dynamic models from £500+VAT. Turnaround is 5 working days once we have everything we need.

Questions we get asked

Does Part O apply to extensions and conversions? No. Part O covers new residential buildings: houses, flats, care homes and student accommodation. Some councils still ask for an overheating check at planning, and we can provide one.

Can we just add air conditioning? Only where passive measures cannot meet the requirement. The report has to show that glazing, shading and openings were considered first.

Are the rules changing? In March 2026 the government said it will carry out a full technical review of Approved Document O. Until it changes, the current edition applies.

What do you need from us? Floor plans and elevations with window sizes and opening types, the site address and orientation, and any noise report or security requirements.

More: Part O overheating assessments, TM59 dynamic overheating assessments, Part O simplified method or TM59?, SAP calculations and EPCs, industry updates

Email your drawings or WhatsApp us for a free quote.