TM59:2026 and Part O answer different briefing questions
A TM59:2026 overheating assessment brief should name its purpose and methodology. Writing “latest TM59” is insufficient when the same project needs building regulations evidence and a wider design review.
CIBSE lists TM59:2026 as active guidance, published in July 2026. Its publisher notice says that TM59:2017 is still required for Part O compliance, while recommending TM59:2026 for industry best practice. However, a new professional edition does not automatically amend Approved Document O.
Use the following checklist to make the purpose, assessment basis and design responsibilities explicit.
Record the purpose before selecting the assessment
| Purpose | What the brief should record |
|---|---|
| Part O evidence in England | Confirm that Part O applies, the intended compliance approach, the relevant Approved Document and the methodology accepted for that purpose. |
| A wider design or resilience assessment | State the client’s question, relevant professional guidance, scenarios, outputs and how the team will use the findings. |
| Both purposes on one project | Identify each deliverable separately. Explain any differences in assumptions and avoid treating one pass/fail statement as both conclusions. |
Approved Document O includes a simplified method and a dynamic thermal modelling method. Its dynamic route sets limits on the use of TM59 and addresses usable overheating measures. Confirm the appropriate route with the building control team early.
Our energy and sustainability guidance explains the wider planning context. A planning statement and a building regulations assessment have different roles.
Give every specialist the same usable design
The most useful coordination question is whether the model represents the design people will actually use. Gather the current drawings, room uses, glazing information, shading proposals and ventilation strategy before the modelling brief is final.
Next, identify decisions that remain open. A window-opening assumption, for example, may depend on noise, security or other practical constraints. Ask the relevant specialists to state those constraints in a form the modeller can use.
- Assign a revision and owner to each input.
- List the rooms and design options the assessment will examine.
- Separate confirmed specifications from provisional assumptions.
- Agree which changes require the model or report to be revisited.
- Keep the final report, design response and supporting drawings together.
Test the handover with one design change
Illustrative example: an early overheating model assumes generous window opening. Later acoustic design restricts that opening on a noisy façade. The team should ask the modeller to review the actual opening arrangement before relying on the original result.
Record the change, the affected rooms and the options still available. Then compare the options on a consistent basis. If the design team selects a different solution, update the drawings and specification as well as the report.
This approach makes the assessment part of the design discussion. It also gives the client a traceable explanation of what the conclusion depends on.
Ask for a report that states its limits
The report should make its purpose, edition, inputs and conclusions easy to find. Also ask for a clear distinction between the assessed design and suggestions that still need development.
Where the project has several assessment purposes, request a short reconciliation note. It should explain which result answers each question and where further work remains. Avoid a cover page that says only “TM59 compliant” without identifying the basis.
For broader coordination, see our Future Homes Standard design briefing. To discuss the environmental interfaces, send ACP the project stage and current assessment brief.
Sources and scope
Sources: CIBSE’s TM59:2026 publisher notice; Approved Document O; and the government’s Part O frequently asked questions. The Part O discussion concerns England. Check the relevant jurisdiction and project circumstances.