Approved Document O: What It Covers and Who It Applies To

Approved Document O is the part of the Building Regulations dealing with overheating in new residential buildings in England. If you are designing or assessing new homes, it is a gate every dwelling has to pass.

What it applies to

Part O applies to new residential buildings. That includes dwellings, residential institutions and student accommodation. A conservatory built as part of a new residential building must also meet it.

It does not apply to:

• Extensions to existing buildings
• Conservatories added after a building is built
• Buildings undergoing a change of use

That last one surprises people. A conversion is not caught by Part O, however warm it gets.

What it asks for

Requirement O1 is about limiting unwanted solar gain in summer and providing an adequate means of removing excess heat, and doing it using passive means as far as reasonably practicable.

That phrase does real work. If you use mechanical cooling, you are expected to show that all practicable passive measures have been incorporated first. Mechanical cooling also carries Part L consequences and must be reflected in SAP calculations.

A home can still count as predominantly naturally ventilated if mechanical ventilation is used for night cooling.

The two routes

You can demonstrate compliance by the Simplified Method, which sets prescriptive glazing and free-area limits, or by the Dynamic Thermal Method, which requires a report showing the dwelling passes a CIBSE TM59 assessment following TM59 section 2.3, along with the Appendix B compliance checklist for the Building Control Body.

We cover how to choose between them separately.

Details that catch people out

Risk locations. High and moderate, determined by postcode. Central Manchester, for instance, takes moderate risk values as a minimum, though higher-risk values may be considered for postcodes listed in Appendix C.

Shading. A due south overhang should fully shade the pane at solar altitudes of 50° and above.

Free area. For a cross-ventilated house in a moderate risk location, the bedroom minimum free area is 4% of floor area. For a 12m² room that is 0.48m². A 2m × 0.5m window opening to 30° gives an equivalent area of about 0.60m², which clears it.

Security. Ground-floor and other easily accessible rooms unoccupied at night have windows, patio doors and balcony doors modelled as closed. Ventilation louvres and other secure openings are excluded from that.

Window handles and escape. Handle height is taken as 650mm when calculating a safe opening angle, and a physical restrictor is not required for that purpose. Escape windows have an opening height of 1100mm above floor level, with a build tolerance of +0/−100mm.

Guarding. Guarding in front of a window can reduce equivalent area and should be assessed case by case. Openings wider than 100mm that form part of the overheating strategy need protection from falling; a window with a fixed 100mm restrictor does not, and its equivalent area is calculated with the restrictor in place.

Part F is not Part O. Complying with Part F for ventilation does not demonstrate that a dwelling will not overheat. They are different requirements answering different questions.

The practical point

Part O rewards early thinking and punishes late discovery. Orientation, aspect, glazing ratio and whether a window can be safely left open at night are all decided long before anyone runs an assessment, and all of them are cheap to change early and expensive to change late.

This is a summary, not a substitute for the Approved Document. Check the current version before relying on any figure here.

AutoTHERMAL runs both Part O routes in one tool. [Join the waitlist](/waitlist).