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02 · Building Regulations · Part O

Overheating assessment

Part O of the Building Regulations requires new homes to be designed so they do not overheat in summer. You show compliance either with the simplified method or with a dynamic thermal model to CIBSE TM59.

DWGuide by Dan Wallis · Updated 27 September 2026

This guide covers the rules in England. See Wales, Scotland and Northern Ireland.

45 overheating assessorsPart O
Who needs oneAnyone building a new home (not extensions or conversions)
Who can do itEnergy assessors or thermal modelling consultants
WhenDesign stage, before building control approves the plans

45 overheating assessors listed across the UK

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Find overheating assessors by region

London (13) South East (6) East of England (9) South West (6) West Midlands (5) East Midlands (4) Yorkshire and the Humber (3) North West (6)

National providers (34)

Regional providers (check they cover your area) (1)

Two ways to comply

Quicker and cheaper

Simplified method

Checks glazing areas, orientation, shading and how much the windows can open, based on whether the site is in a moderate- or high-risk location.

More design freedom

Dynamic modelling (TM59)

A computer model of the home through a hot summer. It is useful for larger windows, flats and higher-risk sites where the simplified method fails.

Design choices that help

  • Keeping large glazed areas off south and west-facing walls, or shading them
  • Windows that open wide enough for cross-ventilation
  • Passive measures first, before relying on any mechanical cooling

In Wales, Scotland and Northern Ireland

This guide describes the rules in England. Building regulations are devolved, so here is how overheating assessment works in the rest of the UK.

Wales

Wales has its own Part O, in force since 23 November 2022. As in England, it covers new homes only, using the simplified method or TM59 modelling.

Scotland

Standard 3.28 has required new homes to limit the risk of overheating since February 2023. The assessment goes in with the building warrant application, using either a simplified method or TM59 modelling.

Northern Ireland

There is no equivalent of Part O. Designers only need to show a basic check on solar gains as part of the SAP calculation.

Common questions

Does my extension need an overheating assessment?

No. Part O only applies to new residential buildings, not extensions, conservatories added later, or changes of use.

Which method should I use?

Try the simplified method first. If the design fails it, for example because of large glazing or a high-risk location, dynamic modelling gives more flexibility.

When is the assessment done?

At design stage, before building control approves the plans, so any changes to glazing, shading or window openings can still be made.

What design choices help a home pass?

Keeping large glazed areas off south and west-facing walls or shading them, windows that open wide enough for cross-ventilation, and passive measures before relying on any mechanical cooling.

New to the jargon? See our glossary of terms, from air permeability to TrustMark.