Approved Document O (Overheating): What Homeowners Need to Know

Approved Document O supports Regulation 40A of the Building Regulations, which states that reasonable provision must be made to limit unwanted solar gain and provide an adequate means of removing heat from dwellings.

Part O came into force on 15 June 2022 as part of the wider 2021 uplift to the Building Regulations alongside Part F (ventilation) and Part L (energy efficiency). It applies to:

• New dwellings (new build houses and flats)
• Extensions to existing dwellings
• Residential buildings undergoing a material change of use

Part O does not apply to existing buildings that are not being materially altered or extended. Replacing like-for-like windows or doors in an existing home does not trigger Part O compliance, even if the new glazing area is larger than the minimum needed.

Well-insulated, airtight modern homes retain heat effectively — which is excellent in winter, but can result in dangerously high indoor temperatures in summer. Large south- and west-facing glazing areas act as solar collectors, admitting heat energy that then has no easy way to escape.

Studies by the Building Research Establishment and others found that a significant proportion of UK dwellings — including new builds — already exceeded overheating thresholds defined by CIBSE. With climate change increasing the frequency and severity of UK heat events, Part O was introduced to design this risk out of new construction.

The Part O methodology assesses the risk of overheating based on:

• Location — London and the South East are higher-risk than northern England
• Orientation — south and west elevations receive the greatest solar gain
• Glazing area — total glazed area as a percentage of floor area
• Solar shading — overhangs, external blinds, and recessed frames reduce solar gain
• Ventilation — cross-ventilation and purge ventilation (opening windows) are the primary means of removing heat

This is the most common scenario where Part O becomes a live issue for homeowners. A rear extension with a full-width glazed rear wall — whether bifold doors, sliding doors, or a combination of fixed glazing and doors — can introduce a very large area of south- or west-facing glass.

Part O compliance is assessed using one of two routes:

1. The simplified method — the Approved Document sets glazing area limits by orientation. If your glazing area stays within these limits, compliance is demonstrated without further calculation. The limits are tightest for south- and west-facing elevations in high-risk locations.
2. The dynamic thermal modelling route — a more detailed calculation (typically using software such as IES VE or TAS) that models the thermal performance of the building hour by hour. This route can demonstrate compliance even where glazing exceeds the simplified limits, if appropriate mitigation measures are in place.

In practice, many rear extensions with large bifold door runs can comply via the simplified method if the glazing area is kept within limits, or via dynamic modelling with appropriate shading and ventilation.

If your design triggers Part O compliance concerns, the following measures are commonly used to reduce overheating risk:

• External shading — a fixed overhang or canopy above glazing is one of the most effective measures; it blocks high-summer sun while allowing lower-angle winter sun to enter
• External blinds or louvres — motorised external blinds are very effective at reducing solar gain; internal blinds are much less effective and Part O gives them limited credit
• Solar control glazing — glass with a low solar factor (g-value) limits the proportion of solar radiation transmitted; there is a trade-off with daylight levels
• Reducing glazing area — the simplest solution; replacing a full-width bifold run with a combination of solid panel and door can bring a design within the simplified method limits
• Cross-ventilation — windows on opposite or adjacent elevations allow through-breezes; this is assessed in the Part O methodology
• Mechanical ventilation with summer bypass — MVHR systems in new builds can be configured for night purging to cool the structure overnight

The simplified method in Approved Document O uses tables of maximum glazing area percentages by orientation and location. The document divides England into two risk zones — a higher-risk zone covering most of London, the Thames Estuary and parts of the South East, and a lower-risk zone covering the rest of England. Within each zone, limits are set for different orientations (north, east, south, west, roof).

The dynamic thermal modelling route assesses overheating against the CIBSE TM59 criterion: the number of hours per year that the operative temperature in living rooms exceeds 26 degrees Celsius, and in bedrooms exceeds 26 degrees Celsius during sleeping hours. If modelling shows the dwelling meets TM59 thresholds, it complies with Part O regardless of glazing area.

Dynamic modelling is typically commissioned where the simplified method would require a significant reduction in glazing area that the client does not wish to make. It adds cost (typically a few hundred pounds to the design fees) but gives more design freedom.

Conservatories that meet the specific definition in the Building Regulations — a thermally separated glazed structure at ground level — are generally exempt from Part O because they are not treated as habitable living space under the regulations. However, if a structure is:

• Heated to the same standard as the house
• Open-plan to the main living space without thermal separation
• Designed as a habitable room (office, dining room, living room)

...then it is not a conservatory for Building Regulations purposes, and Part O will apply. Modern garden rooms and lifestyle extensions are frequently in this category, and designers and homeowners are sometimes surprised to discover that compliance is required.

Your building control body will require evidence of Part O compliance as part of the building regulations application. This is typically provided by:

• A completed simplified method compliance table (often produced by your architect or designer)
• A dynamic thermal modelling report (if the simplified method route is not taken)

Building control will review the methodology, check that the glazing areas and shading measures match the actual design, and confirm compliance before issuing a completion certificate. If you are using a structural warranty provider alongside local authority building control, they may have their own requirements around overheating evidence.