Building Regulations Part O and overheating

Approved Document O (Overheating) was introduced to address the growing risk of excessive internal temperatures in UK homes, driven by climate trends, high levels of insulation and large glazed areas. It applies to new dwellings in England and requires reasonable provision to limit solar gain and promote passive cooling. Roof lights — which receive direct sunlight for much of the day when on south- or west-facing slopes — can materially affect whether a dwelling passes the overheating assessment.

This article explains Part O in general terms. Overheating analysis is technical and edition-specific; your energy assessor or sustainability consultant will run the applicable method for your project.

Why overheating became a regulatory issue

Well-insulated, airtight homes retain heat efficiently in winter but are vulnerable to summer overheating if solar gains are not managed. Bedrooms and nurseries are priority spaces because high night-time temperatures affect sleep and health. Facade glazing has long been scrutinised; roof glazing can be equally significant because it faces the sky and receives solar radiation at high incidence angles in summer.

Part O sits alongside Part L (which may encourage larger windows for daylight and passive solar heating) and Part F (ventilation). The three must be balanced in the design.

Compliance routes in Approved Document O

The document offers two main paths:

Simplified method — a checklist approach using glazing area as a proportion of floor area in each room, adjusted for orientation, shading from overhangs or balconies, and whether glazing is in the roof or facade. Rooms that fail the criteria require mitigation or must pass the dynamic modelling route instead.

Dynamic thermal modelling (CIBSE TM59) — a simulation of internal temperatures using weather data, occupancy patterns, ventilation behaviour and shading. More flexible but more expensive. Openable roof lights, curtains, blinds and g-values are modelled explicitly.

Most small projects attempt the simplified method first; complex dwellings or those with extensive roof glazing often move to TM59.

Roof light-specific considerations

Roof lights affect Part O in several ways:

• Glazed area — each unit adds to the numerator in the glazing-to-floor ratio. Multiple large roof lights in a top-floor bedroom increase overheating risk classification.
• Orientation — a roof light on a south-facing slope receives more summer solar gain than one on a north-facing pitch. Flat roof lights on horizontal roofs receive strong midday sun.
• g-value — solar energy transmittance. Lower g-values reject more solar heat. Solar control coatings on outer panes are a common specification response for roof glazing in sensitive rooms.
• Shading — external shading (overhangs, brise soleil, louvres) is more effective than internal blinds because it blocks heat before it enters the glass. Roof lights lack the eaves shading that protects vertical windows on many elevations.
• Ventilation — openable roof lights support stack ventilation and night cooling, credited in dynamic modelling and sometimes in the simplified method’s mitigation options.

Mitigation strategies for specifiers

If initial calculations show overheating risk in a room with roof lights, common responses include:

1. Reduce roof glazing area or relocate units to lower-risk rooms (halls, bathrooms).
2. Specify solar control glazing with a lower g-value on south- and west-facing units.
3. Provide external shading — motorised blinds integrated into the roof light, adjacent shading structures or architectural overhangs on lanterns.
4. Increase openable area for cross-ventilation and night purge, coordinated with Part F.
5. Move to TM59 modelling to demonstrate compliance with realistic occupancy and operation assumptions.

Interaction with planning and design quality

Part O is a building regulation, not a planning condition, but planners and design review panels increasingly scrutinise glazing proportion and climate resilience. Early architectural decisions on roof form and roof light count are harder to reverse at specification stage. Involve the energy assessor before the roof layout is fixed.

What Part O does not cover

Approved Document O in its current form targets new dwellings in England. Extensions, commercial buildings and renovations are generally outside its scope, though good practice on overheating still applies. Other nations in the UK may adopt different approaches.

Product data you will need

Your assessor needs g-value, U-value and light transmittance for each roof light specification, plus openable area if ventilation credit is claimed. Manufacturer declarations for the exact unit size avoid generic assumptions that skew the calculation.

Discuss orientation, room use and glazing area with your supplier when configuring custom roof lights — solar control options and opening configurations can be selected to support Part O compliance.

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