Building Regulations Part O sets legal limits on how warm a new home can get in summer
If you are planning a new home or a major conversion, you need to know about Part O of the Building Regulations. This regulation came into force on 15 June 2022 and applies to all new residential buildings in England (GOV.UK, 2026).
Part O is a mandatory regulation limiting summer temperatures in new homes. You must comply using either a Simplified Method or Dynamic Thermal Modelling. Failure blocks building control approval. Check the rules with your designer now.
- Part O limits indoor temperature to 26°C in summer for new homes.
- Two routes: Simplified Method or Dynamic Thermal Modelling.
- Simplified Method caps south-facing glazing at 22% of floor area.
- Glazing must have a g-value of 0.4 or less under the Simplified Method.
- Failure to comply can stop your building control approval process.
- Building Regulations Part O sets legal limits on how warm a new home can get in summer
- Two compliance routes exist for Part O – the Simplified Method and the Dynamic Thermal Modelling Method
- The Simplified Method relies on specific limits for glazing area, solar gain, and ventilation
- Quick numbers – key Part O limits and thresholds for homeowners and builders
- Part O applies to new homes, but also to certain conversions and extensions that create new habitable rooms
- The direct answer to "building regs part o" – it is a legal requirement to prevent overheating in new homes, using either a simplified checklist or a computer model
- To verify Part O compliance, you must use a certified professional with the correct accreditation
The direct answer is that Part O is a legal requirement that limits how hot a new home can become in summer, using either a simplified checklist or a computer model. It is designed to reduce the risk of overheating and the need for mechanical cooling, protecting occupant health and comfort.
The regulation applies to new dwellings, new buildings containing dwellings, and certain changes of use (e.g., offices to flats). Compliance is mandatory for building control approval; failure to meet Part O can stop a project (GOV.UK, 2026).
Two compliance routes exist for Part O – the Simplified Method and the Dynamic Thermal Modelling Method
You have two ways to demonstrate that your new home meets the overheating standards. The Simplified Method uses a set of pre-defined criteria (e.g., window size, glazing type, shading) to demonstrate compliance without complex modelling (GOV.UK, 2026).
The Dynamic Thermal Modelling Method uses approved software to simulate the building’s thermal performance over a year, considering local climate data. The Simplified Method is quicker and cheaper but may not be suitable for all designs, especially in areas with high overheating risk. The Dynamic Method offers more flexibility for non-standard designs but requires specialist input and costs more (CIBSE, 2026).
The Simplified Method relies on specific limits for glazing area, solar gain, and ventilation
If you use the Simplified Method, your design must meet three main limits. The maximum glazing area is limited to a percentage of the floor area, such as 22% for south-facing rooms (GOV.UK, 2026).
Glazing must have a minimum solar heat gain coefficient (g-value), typically ≤0.4 for large windows. The g-value measures how much solar radiation passes through the glass. Cross-ventilation must be achievable through openable windows or vents, with minimum free area requirements. Shading such as external blinds or brise-soleil can reduce solar gain, but internal blinds alone are not counted (GOV.UK, 2026).
Quick numbers – key Part O limits and thresholds for homeowners and builders
| Parameter | Part O Simplified Method | Part O Dynamic Modelling Method | Typical pre-Part O design |
|---|---|---|---|
| Glazing area limit (% of floor area) | 22% (south-facing) | No fixed limit; modelled outcome | Often 25–30% |
| g-value limit | ≤0.4 (large windows) | No fixed limit; modelled outcome | Often 0.5–0.7 |
| Minimum free ventilation area (mm² per m² floor) | 1/20th of floor area (cross-vent) | Modelled requirement | Often 1/30th of floor area |
| Maximum internal temperature threshold | 26°C (living rooms), 23°C (bedrooms) | Modelled to stay below these thresholds | No legal limit |
Source: Approved Document O (2021 edition incorporating 2023 amendments), DESNZ, 2026.
Part O applies to new homes, but also to certain conversions and extensions that create new habitable rooms
New-build homes must comply fully. The regulation does not apply to existing homes unless they are being extended or converted in a way that creates a new dwelling. Changes of use such as from commercial to residential must meet Part O if the building is not already compliant (GOV.UK, 2026).
Extensions that include a new habitable room, such as a bedroom, may trigger Part O, but a simple replacement window does not. The regulation does not apply to single-storey extensions of less than 30m² unless they are part of a larger scheme (GOV.UK, 2026). What counts as a habitable room under Building Regulations
It is a legal requirement to prevent overheating in new homes, using either a simplified checklist or a computer model
Homeowners do not need to calculate anything themselves. Your architect or builder must demonstrate compliance to building control. The regulation is not optional; it is enforced by local authority building control or approved inspectors (GOV.UK, 2026).
Compliance can be achieved with passive measures such as shading, ventilation, and window design without needing air conditioning. If mechanical cooling is used, it must meet energy efficiency standards such as a minimum Seasonal Energy Efficiency Ratio (SEER) (GOV.UK, 2026). Air conditioning vs passive cooling costs
To verify Part O compliance, you must use a certified professional with the correct accreditation
The designer, an architect or engineer, must be competent in overheating analysis, typically via a professional body such as RIBA or CIBSE (CIBSE, 2026). The installer of any mechanical cooling must be F-Gas registered if using refrigerants (GOV.UK, 2026).
Building control officers or approved inspectors will check the final compliance report. No specific MCS or TrustMark scheme covers Part O alone. For the Dynamic Modelling route, the software user should have appropriate training, such as CIBSE TM59 or TM52 (CIBSE, 2026).
Frequently Asked Questions
Part O is a legal requirement in England that limits how hot a new home can get in summer. It came into force on 15 June 2022 and applies to new dwellings and certain conversions (GOV.UK, 2026).
No, Part O does not apply to extensions. It only covers new dwellings, new buildings containing dwellings, and changes of use such as offices to flats (GOV.UK, 2026).
The two routes are the Simplified Method and the Dynamic Thermal Modelling Method. The Simplified Method uses pre-defined limits on glazing and ventilation. The Dynamic Method uses approved software to simulate annual thermal performance (CIBSE, 2026).
Under the Simplified Method, the maximum glazing area is limited to 22% of the floor area for south-facing rooms. Glazing must also have a solar heat gain coefficient (g-value) of 0.4 or less (GOV.UK, 2026).
Yes, but Part O is designed to reduce the need for mechanical cooling. If you use it, you must still meet the overheating limits and demonstrate that passive measures have been prioritised (GOV.UK, 2026).
