Window to wall ratio and the overheating conflict

Glazing area is the one design variable that gets pulled hard in two directions. Daylight provision criteria reward more glass, Approved Document O penalises it, and on a single aspect south-facing flat the two sets of numbers can be irreconcilable without changing the plan. The sooner you test both, the fewer rounds of redesign you pay for, because the window to wall ratio is fixed by the elevation long before anyone runs a thermal model.

The conflict is real rather than theoretical. A 2.1m high window across a 3.6m wide living room gives a daylit space that will satisfy most assessors. The same window on a south or west elevation, in a dwelling that cannot be cross-ventilated, will usually fail the overheating criteria without external shading.

Why more glazing is not always better

Solar gain through glass scales with area and with the total solar energy transmittance of the glazing, usually written as the g-value. Daylight scales with area and with light transmittance. The two properties are linked by the glass coating, and the ratio between them, the selectivity, is what the glazing industry sells on. A clear double glazed unit transmits a lot of both. A solar control unit cuts the heat gain, and it cuts the light as well, just less sharply.

So the designer has three levers rather than one: the area of glass, the properties of the glass, and the amount of sky each window can actually see. On a deep plan room with a balcony above the window, the third lever is already working against you, because the balcony soffit removes most of the visible sky from the lower part of the opening while doing very little about summer gain from the west.

Deep reveals help both. A window set 300mm back in the facade loses a little daylight and a lot of high-angle summer sun. That is the least costly move available, and it is usually lost in value engineering.

Where Part O pushes glazing down

Approved Document O gives two compliance routes for residential buildings. The simplified method sets maximum glazing areas as a proportion of floor area, together with minimum free opening areas for removing excess heat, and it varies those limits by orientation group, by whether the location is high risk or moderate risk, and by whether the dwelling is cross-ventilated. A single aspect dwelling gets the tighter numbers on both counts.

The second route is dynamic thermal modelling to the CIBSE methodology. That route allows a higher glazing area where the modelling shows the criteria are met, and it is the normal choice for apartment schemes in urban locations. It costs more and it needs the facade design reasonably settled.

Two constraints on window opening do most of the damage. Where external noise would prevent windows being opened at night, Approved Document O does not allow the design to rely on them for night cooling, and it sets threshold noise levels at which that applies. Where guarding or security means a window cannot be left open on a ground floor or at a low level, the same problem arises. Both are common on the sites where the daylight case is also hardest, next to a main road or in a dense urban block.

What the daylight criteria ask for

Internal daylight for new homes is assessed against BS EN 17037 and its UK national annex, with BRE 209 (2022) giving the methodology most assessors follow. The national annex recommends minimum target illuminances of 100 lux in bedrooms, 150 lux in living rooms and 200 lux in kitchens, achieved over at least half of the assessment grid for at least half of the daylight hours in the year.

Those numbers are not hard planning policy in themselves. What makes them matter is that officers and inspectors use them, and a scheme that misses them in a large proportion of rooms attracts a refusal reason about the quality of the accommodation. The approach to assessing daylight in a proposed building's own rooms differs from assessing the effect on neighbours, and the two should not be conflated in a single report.

The orientation column is what the table cannot show. Horizontal shading works on a south elevation, where the sun is high when it matters, and does almost nothing on a west elevation, where late afternoon sun arrives at a low angle through the window. Fins and screens are the west elevation answer, and they cost more daylight because they block sky as well as sun.

Modelling both at the same time

Running the two assessments in sequence, with different consultants, on different versions of the drawings, is how schemes end up with a facade that satisfies neither. Commission them against the same model and the same room schedule, and agree at the outset which rooms are the worst cases for each.

A workable sequence:

1. Fix the plan depth, aspect and ceiling height, and identify the single aspect units and their orientations
2. Run a first pass daylight check on the worst rooms at the proposed window sizes
3. Run a first pass overheating check on the same rooms with realistic opening areas and any noise constraint applied
4. Resolve the conflict with reveal depth, shading geometry and glass selection before changing window sizes
5. Only then adjust glazing area, and recheck both
6. Record the final glazing specification, free opening areas and shading geometry in one schedule

Step 3 is the one that gets skipped, because the overheating consultant is usually appointed at technical design stage rather than at planning. On a scheme with many single aspect units, getting an early indicative run is worth the fee even though the formal assessment will be redone later.

An illustrative example, for method rather than for numbers. A single aspect west-facing one bedroom flat on a road with night-time traffic noise cannot rely on open windows after 10pm. The overheating strategy therefore depends on mechanical ventilation with a summer bypass, and the glazing has to carry a lower g-value. Dropping the glass to a lower g-value costs some light transmittance, so the window head is raised by 200mm to recover the daylight, which is only possible because the floor to ceiling height was set at 2.6m rather than 2.4m at the start.

Presenting the balance to the officer

Planning officers are not assessing Part O compliance, which is a Building Regulations matter determined by the building control body. They are assessing the quality of the accommodation, the appearance of the elevation and, in London, compliance with the cooling hierarchy in the London Plan. What they need from you is an explanation of why the elevation looks the way it does.

Put the reasoning in the design and access material and in the Energy Statement, and keep the two consistent. If the shading fins on the west elevation are there for overheating reasons, say so, because otherwise they read as arbitrary and an officer may ask for them to be removed on appearance grounds. Where an Overheating Risk Assessment has been prepared, summarise its conclusions and the design measures it depends on rather than appending it without comment.

Where the scheme also affects neighbouring windows, a Daylight and Sunlight Assessment deals with that separately. Keep the internal and external analyses in different documents, because mixing them makes both harder to follow at committee.

Conditions on the glazing specification

Councils increasingly condition glazing performance, and the wording matters. A condition requiring a named product is fragile, because the product gets superseded between permission and construction and you then need a formal amendment. A condition requiring the glazing to achieve a stated maximum g-value and minimum light transmittance, with the shading geometry shown on approved drawings, gives the same control and survives procurement.

• Condition performance values and shading geometry, not product names
• Ask for the free opening areas to be shown on the approved window schedule rather than described in text
• Resist conditions that require mechanical cooling to be designed out after permission, because the overheating strategy may depend on it
• Where a condition requires post-completion verification, agree what evidence will satisfy it before you accept the wording

And if the glazing has to change between permission and construction, deal with it properly. A reduction in window size that affects the appearance of the elevation is not always a non-material amendment, and a facade that drifts during technical design is a common source of enforcement correspondence on apartment schemes.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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