Part O has been law since June 2022, but more than two years on it remains one of the most commonly misunderstood parts of the Building Regulations. The compliance routes look straightforward on the page, and the simplified method seems designed for ordinary projects. In practice, most schemes that get submitted for sign‑off on the simplified route shouldn’t be. The result is rework, delay, and — increasingly — buildings that pass on paper but get complaints in their first summer.
What Part O is, briefly
Part O sets minimum standards for limiting overheating in new residential buildings. It applies to anything with a sleeping area: houses, flats, residential institutions, student accommodation. Two compliance routes are available: the simplified method (a spreadsheet check based on glazing area, opening area and shading) and dynamic thermal modelling (typically a TM59 assessment using software such as IES VE).
Where the simplified method works
The simplified method works well for low‑risk dwellings. That usually means: dual‑aspect units, generous opening area, modest glazing ratios, rural or suburban sites without significant noise, pollution or security constraints, and no unusual shading. Many rural house‑builder projects fit this profile and pass the simplified check comfortably.
Where it doesn’t
The method falls down on the schemes where overheating is most likely to be a problem. Single‑aspect urban flats. Schemes near rail lines, busy roads or industrial sites where windows can’t realistically be assumed openable overnight. Buildings with large south or west‑facing glazing. Schemes with deep balconies that change solar gains in ways the spreadsheet doesn’t capture. The simplified method assumes idealised conditions — free opening, no noise constraint, conventional form — and most urban residential schemes don’t meet those assumptions.
The cost of using the wrong route
Submitting a simplified check for a building that needed TM59 is the single most common reason we see schemes referred back at building control. The fix is straightforward: a TM59 assessment checks the operative temperature in each occupied space across a CIBSE design summer year and compares it against the static and adaptive thresholds in Part O. But by the time the building control surveyor has flagged it, the M&E design is often already locked in, ventilation strategies have been priced, and changing course costs time and money.
Designing for Part O from the start
Three decisions at concept stage dramatically reduce the risk of an overheating problem later. First, orientation and glazing ratio. Limiting south and west‑facing glazing on single‑aspect units, or specifying solar control glass, makes both compliance routes easier. Second, openable area. Window restrictors fitted retrospectively for safety almost always reduce opening area below the modelled assumption — if the design accounts for restrictors from day one, the assessment is reliable. Third, ventilation strategy. Mechanical cooling, MVHR with summer bypass, or hybrid strategies all change the modelling significantly; the time to decide is at RIBA Stage 2, not Stage 4.
The lender and warranty question
It’s also worth knowing that some warranty providers and lenders are now asking for TM59 evidence on schemes that would technically pass the simplified route, particularly for build‑to‑rent and PRS schemes where the developer carries operational risk. A simplified compliance check satisfies building control. It may not satisfy the people writing the cheques.
When to bring in an overheating consultant
As early as possible. A half‑day review at concept stage — looking at orientation, form, glazing and ventilation strategy — typically tells the design team which compliance route is realistic and what design moves protect against overheating risk. That conversation costs almost nothing. The retrospective fix, after planning, can run into hundreds of thousands of pounds on a typical scheme.
If you’re working on a residential scheme and aren’t certain which Part O route is right, get in touch. We do early‑stage reviews regularly and the answer is usually clear in an hour.
