Part O Simplified Method or TM59?

Part O Simplified Method or TM59: Which Do You Need?

Part O of the Building Regulations limits overheating in new homes. Every new house, flat, care home and student residence in England has needed an assessment since June 2022. There are two ways to comply: the simplified method and a TM59 dynamic model. The simplified method is quicker and cheaper. TM59 gives far more design freedom. We check which one your design needs before you pay for either.

The short answer

• Simplified method if the homes have moderate glazing, cross-ventilation and windows that can be left open at night. From £150+VAT per home.
• TM59 if the design is heavily glazed or single-aspect, the site is noisy or polluted, the block has communal heating, or it is a major scheme in London. From £500+VAT.

How the simplified method works

It checks two things for each home:

• Glazing. The glazed area is limited as a percentage of floor area. The limit depends on which way the most glazed side faces and whether the home has cross-ventilation.
• Openings. Windows and doors have to provide a minimum free area so heat can escape.

Homes in the high-risk area, which covers much of London, have tighter limits and also need shading or solar-control glass.

When you will need TM59

• The design does not pass the simplified method
• Heavily glazed designs, large sliding doors or rooflights
• Single-aspect flats with no cross-ventilation
• Sites where noise, security or air pollution mean bedroom windows cannot be left open at night
• Blocks of flats with communal heating pipework running through corridors
• Major planning applications in London
• Care homes and student accommodation

How TM59 works

We build a dynamic thermal model of the building and run it hour by hour against summer weather data. Living rooms, kitchens and bedrooms are tested against limits on how many hours they can be too warm between May and September. Bedrooms have an extra night-time limit of 26°C. Where a room fails, we set out the fix: glazing specification, shading, opening areas or ventilation rates. Mechanical cooling is the last resort.

Side by side

• What it checks. Simplified: glazing area and opening area. TM59: hourly temperatures in every room.
• Design freedom. Simplified: tight limits. TM59: much more.
• Price. Simplified: from £150+VAT per home. TM59: from £500+VAT.
• Turnaround. 5 working days for either, once we have everything we need.

Changes on the way

CIBSE published a new edition of TM59 in July 2026, with a new bedroom test and updated weather data. Part O still refers to the 2017 edition, and the government has announced a full technical review of Part O. Until that is finished, the current edition applies. We model to the edition your building control body or council asks for and state it clearly in the report.

Questions we get asked

Does Part O apply to extensions and conversions? No. Part O covers new residential buildings. Some councils still ask for an overheating check at planning, and we can provide one.

Can we start with the simplified method and switch later? Yes. If the design does not pass, we tell you what to change first: glazing, shading or opening area. If none of that suits the design, a TM59 model gives more freedom.

Can we add air conditioning to pass? Only where passive measures are not enough. The report has to show those were considered first.

More: Part O overheating assessments, TM59 dynamic overheating assessments, overheating overview, how much SAP calculations cost

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