Part O Overheating Assessments in London and Nationwide

Part O overheating calculations by the simplified method

We carry out Part O overheating assessments for new homes in London and nationwide. Part O of the Building Regulations limits overheating in new homes in England, and every new house, flat, care home and student residence has needed an assessment since June 2022. A simplified overheating assessment is the quickest and cheapest route, and we tell you early if your design will need a dynamic model instead.

How the Part O 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.

Overheating assessments in London: the high-risk area

Homes in the high-risk area, which covers much of London, have tighter glazing limits and also need shading or solar-control glass. We check which area your site is in before we start.

When the simplified method will not work

• 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

These need a TM59 dynamic overheating assessment. We check this before we start, so you do not pay for two reports.

What we need from you

• Floor plans and elevations with window sizes and how each window opens
• The site address and orientation
• Any noise report or security constraint, such as ground-floor bedrooms

Turnaround: 5 working days once we have everything we need. Already need SAP calculations? We can price both together.

Free Part O checklist. The 7 things we check first, on one page. Email [email protected] with "CHECKLIST" in the subject and we'll send it to you.

Noise, security and safety

Part O assumes windows are opened to cool the home, so it also checks they can be. Bedroom windows are not counted as open at night where noise in the bedroom would be above 40 dB LAeq averaged over 11pm to 7am, or above 55 dB LAFmax more than ten times a night. Openings also have to be safe to leave open, with guarding where there is a risk of falling and secure openings where they are easy to reach from outside.

Your Part O overheating report

Our Part O overheating report sets out the method, the glazing and opening areas for each home and the checks against the limits. If the design changes before completion, we update it.

Our experience

We have completed more than 50 overheating assessments. A recent one: three new terraced houses in Slough that passed by the simplified method, so no dynamic model was needed.

Questions we get asked

How much does a Part O overheating assessment cost?
From £150+VAT per home, depending on the size and number of homes. Send us the drawings for a free quote. We turn it around in 5 working days once we have everything we need.

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.

What if my design does not pass?
We tell you what to change: smaller or different glazing, shading, or more opening area. If none of that suits the design, a dynamic model gives more freedom.

Is Part O changing?
Not yet. The government confirmed a full technical review of Part O in its Future Homes Standard consultation response in March 2026. Until new guidance is published, the current edition applies.

Guides: Part O simplified method or TM59: which do you need?, why new homes fail Part O (and how to fix it)

Send us your drawings by email or WhatsApp and we will come back with a free quote and a turnaround time.