Establish whether Part O applies
Approved Document O concerns new residential buildings in its specified categories. The fact that an existing room is being enlarged or a loft is becoming habitable does not, by itself, make the whole project a new residential building for this requirement. Confirm the scope before commissioning an assessment under the wrong route. [2]
Planning requirements are separate. In London, the London Plan's housing design provisions address matters including aspect, ventilation and overheating. A project outside London should be assessed against its own planning requirements, not the London Plan by default. [3] [4]
Limit heat gains and provide a usable way to remove heat
The guidance addresses both limiting unwanted solar gains and removing excess heat. Glazing area, orientation, shading and the available ventilation work together. Adding cooling equipment should not be treated as the first answer to an avoidable design problem: the requirement places limits on relying on mechanical cooling where sufficient heat can be removed without it. [2]
Ask the designer to explain the summer strategy in plain terms. Identify which windows or other openings are expected to be used and when, rather than relying on a model that assumes unrestricted opening at all times.
Two assessment approaches
Approved Document O provides a simplified method and a dynamic thermal modelling route, with the latter referring to the relevant CIBSE methodology and the document's additional requirements. The appropriate route depends on the proposal and the limitations of the method; a generic statement that the building has opening windows is not an assessment. [2]
For either route, the assumed construction and openings need to match the specification that will actually be built. Changes to glazing or shading should be assessed before they are accepted as substitutions. [5]
Noise, security and safety can change the answer
The guidance requires the overheating strategy to consider whether openings are usable, including constraints from noise, security and protection from falling. A window that cannot reasonably be left open at night may not provide the assumed cooling in practice. [2] [6]
Coordinate these constraints before the glazing order, not after an energy assessment has been completed. Part L heat-loss compliance and Part F ventilation requirements do not replace the separate overheating assessment. [7] [8]
Handover and future editions
Provide clear instructions on the features occupants need to operate and retain the final assessment. The companion energy and ventilation changes due from 24 March 2027 should trigger a coordination review, but their publication does not mean that Part O automatically extends to every existing-home alteration. [2] [9]