Design & Construction (RIBA 4-5)

TM59:2026 | The Latest Overheating Guidance for Residential Developments

Published on : 15th Oct 2026

Design & Construction (RIBA 4-5)

TM59:2026 | The Latest Overheating Guidance for Residential Developments

The release of CIBSE TM59:2026 introduces significant changes to the way overheating risk is assessed in new homes and major residential refurbishments. The updated methodology places greater emphasis on passive design strategies and future climate resilience, encouraging design teams to address overheating risk earlier in the design process.

Key Changes in TM59:2026

The most significant update is the introduction of a formal three-stage assessment process.

Designs must first demonstrate acceptable performance using unconstrained window-opening conditions and future weather files representing the 2050s climate. Only after this baseline assessment can real-world constraints, such as noise, security and air quality restrictions, be introduced. Enhanced mechanical ventilation and, where necessary, mechanical cooling are then considered.

Other notable changes include:

Adoption of the 2025 CIBSE Weather Data release.
Revised night-time bedroom overheating criteria.
A mandatory pass/fail assessment for communal areas.
New guidance for modelling ceiling fans and home office spaces.
Expanded modelling and reporting requirements for ventilation, cooling, window openings and operational constraints.

Together, these changes encourage a more robust and future-focused approach to overheating assessment.

What Does This Mean for Design Teams?

TM59:2026 places greater importance on passive design measures from the outset. Overheating can no longer be addressed primarily through mechanical solutions later in the design process.

This means greater consideration may be needed for:

Glazing strategy and solar control performance.
External shading.
Building orientation.
Natural ventilation opportunities.
Window opening restrictions resulting from noise, air quality or security requirements.
Ventilation system performance.

Projects with extensive glazing, limited shading or constrained natural ventilation may require design changes earlier in the programme to achieve compliance.

The updated methodology also reinforces the need for close coordination between architects, façade consultants, acousticians and M&E designers. Changes to key assumptions, such as window restrictors, opening configurations, shading strategies or ventilation systems, may require assessments to be revisited.

TM59:2026 and Approved Document O

Importantly, TM59:2026 does not replace the overheating assessment methodology currently referenced by Approved Document O 2021.

For Part O compliance, TM59:2017 and the associated 2016 CIBSE weather files remain the required basis for dynamic thermal modelling, alongside the additional requirements defined within Approved Document O.

However, TM59:2026 may still be required through planning policy, client requirements or project-specific specifications. In some cases, developments may need to demonstrate both Part O compliance and alignment with the latest CIBSE guidance.

To avoid uncertainty later in the design process, project teams should agree at an early stage:

The purpose of the assessment.
The applicable TM59 edition.
The weather dataset being used.
The relevant compliance criteria.

TM59:2026 reflects the industry's growing focus on future climate resilience and overheating risk. By prioritising passive design and testing performance against future weather conditions, the guidance encourages more resilient residential developments and earlier design-stage decision making. For developers and design teams, overheating assessments are increasingly becoming a core design consideration rather than a late-stage compliance exercise.

Need Support with TM59 Assessments? 📧📞

Energist provides TM59 overheating assessments, dynamic thermal modelling and practical design advice to support residential developments from planning through to construction. Our specialists can help identify overheating risks early, assess compliance requirements and develop practical solutions that support both performance and project viability.

Contact our team to discuss your project requirements.