Part O overheating check for a new home, and the drawings it changes
Part O of the Building Regulations asks new homes to limit unwanted solar gains in summer and to be able to remove heat. We run the simplified method from your drawings, tell you what passes, and change the drawings where glazing, ventilation or shading must move. The fee is a fixed £175, no VAT is charged, and our own team does the work. If your scheme needs full dynamic thermal modelling instead, we quote that separately in writing before anything starts.
Fixed fee from £175 · all prices
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What Part O asks of a new home
Requirement O1 of Schedule 1 to the Building Regulations says reasonable provision must be made to limit unwanted solar gains in summer and to provide an adequate means to remove heat from the indoor environment. Approved Document O is the government's guidance on how to meet it. It takes effect on 15 June 2022 for use in England, with transitional arrangements for work already submitted before that date.
Passing is shown in one of two ways: the simplified method, or dynamic thermal modelling. This page is about the first, which is the one that works from a set of drawings and a few tables, and which we do ourselves.
Part O Overheating Check
from £175
Every price is the full fixed fee you pay. No VAT is charged. Council application fees are paid to the council separately.
What you get
- Your scheme sorted into its Part O risk category, by location and cross-ventilation
- Glazing areas checked against the limits for the whole home and its most glazed room
- Openable areas checked against the minimum free areas for removing heat
- Shading specified where the home is in the high risk location
- The drawings amended to match, so building control sees one consistent set
- A written note if the simplified method does not suit and TM59 modelling is needed
- Send your drawings
Plans, elevations and window schedule, with the address, so we can place the home in its risk category.
- Run the simplified method
Glazing areas, openable areas and shading are checked against the tables in Approved Document O.
- Change the drawings
Glazing, ventilation openings and shading are amended where the check asks for it, and the sheets re-issued.
- Written result
You get a short note of what passes, what changed and, if needed, why TM59 modelling is the route.
Which building work it covers
The guidance says it applies to new residential buildings only. That takes in dwellings, which means houses and flats, and residential buildings such as student accommodation and care homes. It covers the shared communal rooms and common spaces of buildings with more than one residential unit. In a mixed-use building it applies to the residential parts and any corridor that serves them.
The wording matters for what it leaves out. An extension to an existing house, or a loft conversion, is not a new residential building, so this Approved Document does not set a Part O test for it. If you are not sure which side your project falls on, send us the drawings and we will say so in writing before you pay for anything. A new house, a replacement dwelling, or a block of new flats is squarely in scope.
Glazing area, window size and position, which windows open and by how much, whether the home has openings on opposite façades, and what shading is fitted are all drawn items.
The simplified method in plain words
The method starts with two questions. Is the home in a moderate risk location or a high risk one? The guidance puts urban and some suburban parts of London in the high risk location, listed in Appendix C of the Approved Document, so many London sites land in the stricter group. And does the home have cross-ventilation, meaning openings on opposite façades? Openings on adjacent façades, as in a corner flat, do not count.
The answers pick a table. The tables cap the glazing area for the whole home and for the most glazed room, and they set minimum free areas for openings so heat can leave. In the high risk location the glazed areas facing between north-east and north-west via the south also need shading, from external shutters with ventilation, glazing meeting a stated g-value and light transmittance, or overhangs on due south façades only.
When the simplified method is not enough
The simplified method is not suitable for a building with more than one residential unit that uses a communal heating or hot water system with significant horizontal pipework, because heat from the pipes warms the common spaces. Some designs also need more glass than the tables allow, and the alternative is dynamic thermal modelling to the CIBSE TM59 method. The report must show the home passes TM59, and the Approved Document puts limits on the modeller's choices.
TM59 modelling is specialist work and is quoted separately, in writing, before we start. It is not inside the fixed fee. We tell you at the start whether we expect your scheme to need it, so you are not surprised halfway.
What the check changes on the drawings
Part O is one of the few Parts where the answer is mostly a drawing decision. Glazing area, window size and position, which windows open and by how much, whether the home has openings on opposite façades, and what shading is fitted are all drawn items. If the check shows a room has too much glazing on its largest façade, the options are less glass, smaller openings, or a different design route, and each is a change to the sheets.
We make those changes as part of the fee, so the Part O result and the drawings agree when they reach building control. The overheating strategy also has to stay usable: safety and reasonable enjoyment of the home are part of the requirement, so we draw the openings and shading with that in mind, and flag any that look unusable in practice.
How it fits with the rest of a new-home submission
A new home needs the full set: building regulations drawings, an energy calculation and, where the design calls for it, structure. The SAP calculations cover Part L energy use, which is a different test from Part O, so passing one does not pass the other. Glazing is where the two meet: more glass can help the energy figures in winter and hurt overheating in summer.
For the whole picture of which report fits which stage, the building regulations reports page sets them side by side, and new build drawings covers the design work if the house is yet to be drawn.
Sources
Every figure on this page, where it comes from and a link to the original.
| Figure | Source | Link |
|---|---|---|
| limit unwanted solar gains in summer | Approved Document O (2021 edition), requirement O1(1): (source wording: "limit unwanted solar gains in summer" and "provide an adequate means to remove heat from the indoor environment") | assets.publishing.service.gov.uk |
| 15 June 2022 | Approved Document O, front matter: (source wording: "takes effect on 15 June 2022 for use in England") | assets.publishing.service.gov.uk |
| new residential buildings only | Approved Document O, paragraph 0.3: (source wording: "applies to new residential buildings only") | assets.publishing.service.gov.uk |
| dynamic thermal modelling | Approved Document O, Requirement O1 intention: compliance can be shown by the simplified method (Section 1) or the dynamic thermal modelling method (Section 2) | assets.publishing.service.gov.uk |
| Appendix C | Approved Document O, paragraph 1.3(b): (source wording: "urban and some suburban parts of London detailed in Appendix C") | assets.publishing.service.gov.uk |
| opposite façades | Approved Document O, paragraph 1.4: cross-ventilation means (source wording: "openings on opposite façades") | assets.publishing.service.gov.uk |
| north-east and north-west via the south | Approved Document O, paragraph 1.9: in the high risk location shading is needed for glazed areas (source wording: "between compass points north-east and north-west via the south") | assets.publishing.service.gov.uk |
| more than one residential unit | Approved Document O, paragraph 1.13: the simplified method (source wording: "is not suitable for buildings with more than one residential unit which use a communal heating or hot water system with significant amounts of horizontal heating or hot water distribution pipework") | assets.publishing.service.gov.uk |
| TM59 | Approved Document O, paragraph 2.4: the building control body should be provided with a report that (source wording: "demonstrates that the residential building passes CIBSE's TM59 assessment of overheating") | assets.publishing.service.gov.uk |
Questions people ask
Does Part O apply to my house extension?
What is the difference between the simplified method and TM59?
Is my London address in the high risk location?
Do you hold an accreditation for overheating assessments?
Will the check change my window sizes?
Photographs: Tim Green from Bradford (CC BY 2.0).
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