Part L addresses energy and carbon performance. Part O addresses overheating in relevant new residential buildings in England. Neither replaces the other, and a pass in one does not demonstrate a pass in the other.
A fragmented workflow often starts with a SAP model based on one window schedule and an overheating assessment based on another. The reports may each look complete, yet the proposed design cannot satisfy both sets of assumptions. The solution is not to merge the regulations. It is to control their shared inputs and make the trade-offs visible to the design team.
Where the assessments overlap
Glazing is the clearest example. Its area, orientation and performance affect winter heat loss, useful gains, summer solar gain and the amount of façade available for an opening. Shading can reduce solar gain but may alter daylight or appearance. Window restrictors can reduce the free opening area available to remove heat.
Fabric, airtightness and ventilation also cross the boundary. Better insulation and lower uncontrolled infiltration support energy performance, while the summer strategy still needs a safe and effective way to remove excess heat. Mechanical ventilation inputs can affect SAP, Part F and the overheating model in different ways.
Start with one controlled geometry set
The site plan, north point, floor plans, elevations, sections and window schedule should agree. Each window needs a stable reference, overall glazed area, opening type and realistic free opening area. Balconies, overhangs, reveals and other shading features should be represented consistently.
On a housing development, orientation and handedness must be mapped to plots. A standard house type does not automatically produce one universal Part O outcome because façades face different directions and adjacent buildings can alter shading or exposure.
Treat usability as a design input
Approved Document O does not reduce overheating design to opening area alone. The strategy must account for whether openings can be used safely and reasonably. Noise, air quality, security, guarding, fall risk and physical restrictions can limit window operation.
An acoustic assessment that assumes closed windows and an overheating report that assumes fully open windows describe incompatible designs. That conflict should be resolved by the relevant specialists before either assumption becomes fixed.
Use a sequence of design responses
First identify the cause: excessive solar gain, limited heat removal, internal gains or a combination. Then test proportionate passive measures such as glazing distribution, external shading, glass performance, opening design or exposed thermal mass where appropriate.
Changing a heating or ventilation system cannot automatically cure a façade-led overheating problem. Equally, reducing every window without considering daylight, appearance and occupant experience is rarely a complete design strategy.
Carry requirements into construction
The final window schedule, glass specification, opening mechanism, restrictors, shading and ventilation system must match the assessed design. Product substitutions should be checked against both workstreams.
Give site and procurement teams a concise schedule of controlled requirements. A compliant model has limited value if the installed windows cannot achieve the stated free area or the specified solar-control glass is replaced.
Confirm location and route
Part O applies to relevant new residential buildings in England only. Establish the applicable Part L edition, Part O route and any planning overheating requirement.
Align the model inputs
Use one controlled set for orientation, geometry, windows, shading, fabric, ventilation and project constraints.
Identify the governing constraint
Explain whether winter energy, summer solar gain, heat removal, acoustics or another requirement is driving the design.
Record accepted measures
Put performance and evidence requirements into the drawings, schedules and change-control process.
Frequently asked questions
Does a good SAP result mean the dwelling will not overheat?
No. SAP and Part O assess different requirements using different methods.
Must every Part O project use dynamic thermal modelling?
No. Approved Document O provides simplified and dynamic routes. The appropriate route depends on the building and constraints.
Is Part O applicable across the UK?
No. Approved Document O applies in England. Other nations and planning authorities have their own requirements.