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Specify fenestration as one coordinated package, not as separate Part L and Part O exercises. For an England project, set a declared whole-window U-value for the actual frame and glass, then check orientation, glazing percentages, solar control and usable opening area against Approved Document O. A window that passes the Part L U-value limit can still fail Part O if it admits too much solar energy or cannot provide enough effective ventilation.
What Part L and Part O each control
Part L addresses energy efficiency: limiting heat loss through the building fabric and controlling the energy used to operate the dwelling. Part O addresses overheating in new residential buildings: limiting unwanted solar gains and ensuring that excess heat can be removed.
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The two requirements interact at the window schedule. Lower U-values reduce winter heat loss, but larger areas of glazing can increase summer gains. Solar-control glass may reduce gains while also reducing daylight. Restrictors can improve safety but reduce the opening area available for purge ventilation. These trade-offs must be resolved before the window order is placed.
The Ministry of Housing, Communities and Local Government describes Approved Document L, Volume 1 as guidance on complying with Part L of Schedule 1 to the Building Regulations and the energy-efficiency requirements for dwellings. Approved Document O is the separate overheating guidance for new residential buildings.
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Confirm which England rules apply
This guidance is England-focused. Approved Document L Volume 1 (dwellings) and Volume 2 (buildings other than dwellings) are listed by GOV.UK as 2021 editions incorporating 2023 amendments. GOV.UK also identifies a separate 2026 edition for work subject to the 2026 standards. Confirm the project’s commencement date, transitional provisions and building type with the building-control team before adopting a window schedule.
- New dwelling: use the new-dwelling Part L limits and the Part O route applicable to the residential scheme.
- Existing dwelling replacement: use the replacement-window provisions and the document’s exceptions for character-sensitive buildings.
- Non-domestic building: Part L Volume 2 applies; Part O is aimed at new residential buildings, so do not transfer dwelling percentages without checking the building regulations route.
Wales, Scotland and Northern Ireland have different regulatory documents. A specification prepared from the England documents should not be presented as a UK-wide requirement.
Part L window and door U-values
For a new dwelling, the limiting standards in Part L Volume 1 Table 4.1 are:
| Element | Limiting U-value |
|---|---|
| Window | 1.6 W/(m²·K) |
| Rooflight | 2.2 W/(m²·K) |
| Door, including a glazed door | 1.6 W/(m²·K) |
These are limiting standards, not necessarily the values that will produce the best SAP or fabric-energy result. The target fabric-energy calculation can require better performance, so the energy assessor may ask for lower U-values or a different balance of glazing, wall and roof performance.
Part L also gives air-permeability limits for new dwellings: 8.0 m³/(h·m²) at 50 Pa and 1.57 m³/(h·m²) at 4 Pa. The airtightness strategy, window installation and test result need to be coordinated rather than treating the window certificate as a substitute for whole-building testing.
Replacement windows in existing dwellings
For replacement windows in an existing dwelling, Table 4.2 requires either a U-value of 1.4 W/(m²·K) or a minimum Window Energy Rating Band B, subject to the document’s exceptions.
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Specify the whole window, not just the centre pane
A Part L window U-value is the performance of the complete thermal element. It includes the glazing, frame and their junction, rather than only the central glass area. Part L Volume 2 explains that a window’s U-value is the combined performance of glazing and frame, assessed using the actual dimensions and configuration, accepted standard dimensions where applicable, or hot-box testing to BS EN ISO 12567.
Ask the supplier to declare the whole-window value for the exact product being ordered. The request should identify:
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- frame material and profile;
- glass build-up and coatings;
- spacer type;
- opening configuration, such as fixed, casement, tilt-and-turn or sliding;
- width, height and number of sashes; and
- any mullions, transoms or vents that alter the thermal calculation.
A centre-pane value can look compliant while the assembled window fails once a conductive frame, a wider spacer or a different size is included. Keep the declaration, calculation or test report, and check that it matches the schedule issued for manufacture.
Part O glazing limits depend on orientation and ventilation
The simplified Part O method does not use one universal glazing percentage. It uses the orientation of the largest glazed façade, the building’s overheating-risk location and whether cross-ventilation is provided. The percentages below are the maximum glazing allowances in Table 1.1 for cross-ventilated buildings.
| Orientation of largest glazed façade | Maximum glazing as percentage of building floor area High risk / Moderate risk |
Maximum glazing in most glazed room High risk / Moderate risk |
|---|---|---|
| North | 15% / 18% | 37% / 37% |
| East | 18% / 18% | 37% / 37% |
| South | 15% / 15% | 22% / 30% |
| West | 18% / 11% | 37% / 22% |
“High risk” and “moderate risk” refer to the location categories used by Approved Document O. Establish the category from the project location rather than assuming that every urban site is high risk or that every rural site is moderate risk.
For a building without cross-ventilation, Table 1.2 is tighter in several orientations. For example, a south-facing largest glazed façade is limited to 11% of building floor area in a high-risk location and 15% in a moderate-risk location; the corresponding limits for the most glazed room are 11% and 15%. Use the complete table that matches the ventilation strategy. Do not apply the cross-ventilation percentages to a single-aspect dwelling.
Measure the areas consistently. Record the floor area used for the calculation, each room’s floor area, the area of glazing and which façade is the largest glazed elevation. A late change from a balcony door to a wider fixed window can move a room or façade over its limit even when the total external wall area appears unchanged.
Solar control: g-value, light and shading
Part L’s U-value does not demonstrate that a dwelling will avoid overheating. Where the simplified Part O limits require additional protection, the solar-control specification must state the relevant optical properties.
In high-risk locations, additional shading is required for glazed areas facing from north-east through south to north-west. Approved options include:
- external shutters that still provide a means of ventilation;
- glazing with a maximum total solar-energy transmittance (g-value) of 0.4 and minimum light transmittance of 0.7; or
- a due-south overhang designed with a 50-degree altitude cut-off.
These are not interchangeable descriptions. If the design relies on glass, obtain the declared g-value and light transmittance for the installed glass and orientation. If it relies on an overhang, retain the geometry showing the 50-degree cut-off. If it relies on shutters, show how they operate while maintaining the required ventilation path.
External shading often performs better against summer gains than internal blinds, but it introduces weathering, maintenance, wind and security considerations. Solar-control glass can reduce cooling demand but may affect colour rendering and daylight. Coordinate the choice with daylight, acoustic, cleaning and user-control requirements rather than selecting a g-value in isolation.
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Calculate usable free area, including restrictors
Part O uses equivalent free area: the opening that occupants can actually use after the sash geometry, restrictor setting, guarding and obstructions are taken into account. Nominal frame size or the area of a closed window is not a ventilation calculation.
| Cross-ventilation case | Minimum total free area | Bedroom minimum free area |
|---|---|---|
| High-risk location | Greater of 6% of floor area or 70% of glazing area | 13% of bedroom floor area |
| Moderate-risk location | Greater of 9% of floor area or 55% of glazing area | 4% of bedroom floor area |
For example, if a high-risk cross-ventilated dwelling has 100 m² of floor area and 30 m² of glazing, the two total-area tests are 6 m² and 21 m²; the larger value, 21 m², governs. That figure still has to be met by the usable equivalent openings, not by adding the full glazed areas of windows that only crack open.
What a 100 mm restrictor changes
A fixed restrictor that limits the opening to 100 mm does not need the paragraph 3.9 guarding standard. The restrictor must, however, be fitted and engaged when the equivalent free area is calculated. A drawing that assumes a fully open sash while the installed restrictor limits it to 100 mm is not an evidence-based calculation.
Openings wider than 100 mm that form part of the overheating strategy need the applicable guarding provisions. Check sill heights, fall protection, guarding strength, child-safety requirements, escape routes and cleaning access together. A restrictor selected for safety can make the free-area target impossible; a larger opening selected for ventilation can trigger guarding or security measures.
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Show the opening width, height, angle, restrictor position and any insect screen, grille or adjacent obstruction on the calculation. If the product has several restrictor settings, identify the setting that will be handed over and maintained.
When the simplified method is not enough
The simplified route is appropriate only when all relevant glazing, shading and free-area conditions are satisfied. If a project exceeds a glazing limit, has an unusual orientation or geometry, cannot provide the required cross-ventilation, or depends on a more complex cooling and shading strategy, use the dynamic thermal method.
The dynamic method requires a report demonstrating that the residential building passes CIBSE TM59 and submission of the Approved Document O compliance checklist to building control. Commission the modeller early enough to influence window sizes, shading and ventilation; modelling a fixed design at tender stage can leave no practical route to compliance.
Mechanical cooling does not automatically remove the Part O design obligation. If cooling is used to pass Part O, the SAP assessment must reflect the installed system. Changing the cooling equipment, controls or operating assumptions can require changes to preserve Part L compliance, so coordinate the SAP and TM59 models with the mechanical specification.
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- Confirm the regulatory route. Record England jurisdiction, dwelling or non-dwelling status, new-build or replacement work, applicable Part L edition and any transitional arrangement.
- Set the window schedule. For each opening, record frame, glazing, spacer, size, configuration, whole-window U-value, g-value and light transmittance where solar control is being claimed.
- Map the Part O inputs. Identify façade orientation, risk category, room floor areas, glazing areas and whether genuine cross-ventilation exists.
- Test glazing percentages. Apply the correct cross-ventilated or non-cross-ventilated table to both the building and the most glazed room.
- Calculate equivalent free area. Use the actual opening geometry, restrictor setting, guarding and obstructions. Apply the total and bedroom minima separately.
- Coordinate safety and performance. Resolve guarding, fall protection, security, escape, acoustic performance, daylight, cleaning and user operation before finalising the hardware.
- Select the assessment route. Use the simplified method only when every stated condition is met; otherwise commission TM59 dynamic modelling and prepare the checklist.
- Close out evidence. Obtain supplier declarations, calculations or test reports, shading details, restrictor information, installation records and the final SAP/TM59 documents for building control and the project file.
Compare window options on more than U-value
When evaluating alternatives, use a schedule that compares the characteristics that affect both regulations and real use:
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- declared whole-window U-value for the actual size and frame;
- g-value and light transmittance, including the effect of coatings;
- opening type and calculated equivalent free area;
- restrictor and guarding requirements;
- orientation-specific shading performance;
- acoustic, security and weathering performance;
- frame sightlines and compatibility with heritage constraints;
- capital cost, maintenance and replacement implications; and
- evidence suitable for SAP, the Part O simplified calculation or TM59.
A product with an excellent centre-pane value but no whole-window declaration is a documentation risk. A low-g glass unit without a stated light-transmittance value cannot substantiate the high-risk shading option. A large opening with no guarding detail is incomplete even if its calculated free area appears generous.
Common specification failures and corrections
Using the glass U-value as the window U-value
Failure: the schedule lists a centre-pane value only.
Correction: request the declared whole-window U-value for the exact frame, spacer, configuration and dimensions, supported by calculation or BS EN ISO 12567 testing where applicable.
Applying one glazing percentage to every elevation
Failure: a generic 20% rule is used for all rooms and orientations.
Correction: identify the largest glazed façade, risk category and ventilation strategy, then apply the matching Part O table to the building and the most glazed room.
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Failure: the free-area calculation uses the nominal sash area while a 100 mm restrictor is installed.
Correction: calculate with the restrictor engaged. If the opening exceeds 100 mm, include the required guarding provisions.
Choosing solar-control glass without optical data
Failure: “solar-control” appears in the specification but no g-value or light-transmittance evidence is provided.
Correction: obtain declared values for the installed glass and demonstrate the relevant Part O shading option.
Leaving TM59 until after procurement
Failure: the design relies on dynamic modelling but windows, shading and cooling are already fixed.
Correction: decide the assessment route during concept design and model the options that can still be changed.
Evidence to hand to building control
The compliance package should allow another person to reproduce the decision. Include the applicable Part L edition and project route; the whole-window U-value schedule; frame, glass and spacer declarations; SAP inputs; orientation and risk-location records; glazing-area calculations; g-value and light-transmittance data where relevant; shading drawings; free-area calculations showing restrictors and guarding; and, where used, the TM59 report and Approved Document O checklist.
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Practical decision rule
Start with the actual window assembly and the project’s orientation and risk data. If the whole-window Part L value, Part O glazing limits, shading requirements and usable free-area requirements all fit, document the simplified route. If any one of those conditions cannot be demonstrated, redesign the fenestration or move to TM59 modelling before manufacture. This sequence prevents a compliant-looking U-value from masking an overheating or ventilation failure.
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