Overheating has been quietly climbing up the design agenda for years, and this month it took a big step. In July 2026, CIBSE — working with Arup and Loughborough University — published a substantially revised second edition of TM59, the standard methodology for assessing overheating risk in homes. It’s been retitled Overheating Risk in Dwellings: A Design Stage Methodology, and it’s less a refresh than a rewrite.
Why TM59 matters to assessors
If you touch Part O, you already know TM59. Approved Document O offers two compliance routes — a simplified prescriptive check, and the dynamic thermal modelling route built on TM59. Where a design can’t pass the simplified method (and plenty can’t), the TM59 model is what decides whether a home overheats on paper. So a new edition of the methodology changes the reference point for a large slice of overheating work.
What’s actually changed
The headline is a shift to a passive-first, three-stage assessment. The 2017 edition was essentially a pass/fail check against the criteria. The 2026 edition makes you demonstrate the design in a strict hierarchy: Stage 1 assesses the dwelling under an unconstrained passive scenario; Stage 2 only allows enhanced mechanical ventilation where real site issues — external noise, poor air quality, security — genuinely restrict opening windows; and Stage 3 treats mechanical cooling as a last resort. It’s the same cooling hierarchy London assessors will recognise from the GLA’s energy guidance.
The number of assessment criteria has grown from two to four. Criterion A covers daytime comfort in naturally ventilated living rooms, kitchens and bedrooms — and now home offices. Criterion C formalises the rules for mechanically ventilated or cooled spaces. And a new Criterion D makes communal circulation areas a mandatory part of compliance, requiring them to stay below 28°C for all but 3% of occupied hours — previously a flagged risk, now a potential fail.
The biggest technical change is the bedroom night-time test (Criterion B). The old rule failed a bedroom if it exceeded 26°C for more than 1% of annual sleeping hours. The new method instead counts overheated nights — a maximum of four between May and September — using mean operative temperature, with sleeping hours shifted to 11pm–8am and thresholds that vary by occupant category. It’s backed by Loughborough University sleep research, and it’s designed to reflect real disruption rather than a blunt hourly count.
A few other changes worth noting: ceiling fans are now formally recognised as a mitigation measure, with specified air speeds and temperature uplifts (though not for sleeping bedrooms or communal circulation, and with rules to stop double-counting in the model); a dedicated home office occupancy profile has been added; the scope now explicitly extends to sheltered housing, care homes, student accommodation, hotels, hospital bedrooms and even retrofit projects; and the reporting requirements are considerably more detailed, expecting you to document and justify every stage, occupant category and ventilation or shading assumption.
The bit that trips people up: the regs haven’t caught up
Here’s the nuance that matters for compliance. TM59:2026 is a CIBSE methodology, not a Building Regulation. Approved Document O 2021 — in force since 15 June 2022 — still references the 2017 edition of TM59. The government confirmed in its March 2026 Future Homes Standard consultation response that Part O will get a full technical review, and adopting the updated TM59 is explicitly on that review’s list — but no revised Approved Document O and no date have been published yet.
So there’s a genuine gap to manage. For strict Building Regulations sign-off, the 2017 methodology remains the referenced document until Part O is revised. But the 2026 edition is now the current industry best practice, and London schemes, planning conditions and many clients’ own standards will increasingly expect it. On a lot of projects you’ll effectively be working to both — which makes it worth reading the new edition now rather than when the regs finally point at it.
What this means for design
The direction of travel is clear and it rewards the same things good assessors already push for: real passive design, effective external shading, sensible glazing ratios and genuine thermal mass, rather than a marginal pass that leans on opening windows at night on a noisy or insecure elevation. Designs built on those assumptions will weather the transition — and any future Part O revision — a lot better than those that don’t.
There’s a knock-on into the energy calculation, too. Where a scheme leans on reduced g-value glazing or mechanical cooling to pass overheating, that choice shows up in the Part L numbers — one more reason to get the fabric and glazing strategy right early, before it’s fighting your SAP result.
If overheating is the sticking point on one of your sites, or you want the fabric, glazing and thermal-bridging side modelled properly so it doesn’t undermine your Part L result, get in touch and we’ll tell you how we’d approach it.
Sources: CIBSE TM59 — Overheating risk in dwellings: a design stage methodology (2026), CIBSE Knowledge Portal · TM59:2026 — Overheating in Residential Buildings; what’s changed, Housebuilder & Developer, 17 July 2026 · Guidance to help tackle overheating risk in homes, Loughborough University, July 2026 · Government Confirms Full Review of Approved Document O (HEM Guide, 9 April 2026, verified against the FHS consultation response) · Approved Document O, Overheating (gov.uk)