Measuring existing windows for a daylight survey

A daylight survey of existing windows needs two different sets of information, and only one of them can be obtained from the street. External geometry, which is what the vertical sky component depends on, is measurable from outside with care. Room geometry and room use, which the no sky line and any internal illuminance calculation depend on, are not. Most of the argument about daylight assessments at committee and at appeal is really an argument about the assumptions made in the second category.

What the assessment needs from each window

Work from a schedule rather than from a drawing, with a unique reference for every window and a row for each of the following.

• Position in three dimensions relative to a site datum, taken to the centre of the glazed opening on the outside face of the wall
• Width and height of the glazed opening, excluding the frame where the calculation requires glazed area
• Reveal depth, because a 215 mm reveal removes measurable sky from a window at a shallow angle
• Cill height above the internal floor level, and head height below the ceiling
• Number of windows serving the same room, and whether the room is single or dual aspect
• Glazing specification where known, and the assumed visible light transmittance where it is not
• Room depth, width and floor to ceiling height
• Room use
• Internal surface reflectances

The first six are measurable externally. The last four are not, and that division should be visible in the schedule itself, with a column recording how each value was obtained: measured, scaled, assumed, or taken from a drawing supplied by the authority.

Measuring from outside when access is refused

Start by asking properly and in writing, because the record of the request matters as much as the measurements. A letter to the occupier and the freeholder, a stated purpose, an offer to share the results, a proposed date, and a follow up. Keep copies. An assessment that states access was refused, with dates, carries more weight than one that states access was not available.

Then measure what you can.

A laser distance meter with an inclinometer, used from a known position, gives window dimensions and heights to an accuracy that is adequate for this work. A total station or a terrestrial laser scan gives better external geometry and a defensible record. Where neither is possible, a rectified photograph scaled against a measured reference on the same facade works, and brick coursing is the most reliable reference on a masonry building because standard coursing is four courses to 300 mm, which lets you scale a facade from a photograph with confidence.

Reveal depth is the measurement people skip and the one that changes results. Measure it at one window on each elevation and apply it to windows of the same type, recording that you have done so.

Setting floor levels without going inside

Internal floor level is the datum for cill height, room depth and the no sky line, and getting it wrong shifts every internal result.

Three external indicators between them usually resolve it. The damp proof course and air brick line give the approximate position of a suspended ground floor. A door threshold, measured against the external ground level, gives the ground floor level directly. And storey heights measured between window cills on successive floors give the floor to floor dimension, from which upper floor levels follow.

Record which indicator you used for each building and state the assumed floor to ceiling height separately from the floor to floor height, because the difference between them is the structural zone and it varies between a Victorian terrace and a 1970s block.

Assuming room depths and room uses

This is where assessments are won and lost.

The sound approach is to make the assumption that is least favourable to your own client, state it plainly, and then test whether the conclusion changes under a different assumption. An assessment that assumes a shallow room because a shallow room produces a comfortable result has no value, and an experienced officer or inspector will say so.

For room depth, the usual starting points are the building depth for a single aspect property, half the building depth where windows appear on both front and rear elevations, and the position of a party wall or a chimney breast visible externally where one is apparent. Terraced housing of a given era has a known plan form, and using it is reasonable provided you say that is what you have done.

For room use, assume the most sensitive use that the window plausibly serves. A ground floor front window on a Victorian terrace serves a living room. A small high window serves a bathroom. Where the use cannot be worked out, assess it as a living room and note that a less sensitive use would improve the result.

Then run the sensitivity test. If the conclusion holds whether the room is 4 m or 6 m deep, say so, and the assumption stops being contentious. If it does not hold, you have found the point the report has to address openly.

Photographic records and how to date them

Photographs are evidence, and undated photographs are not.

Record for each image the date, the time, the camera position described well enough to repeat, and the focal length. Keep the original files with their metadata rather than only the images pasted into the report. Photograph each assessed facade in full, each window type in close up, the reveal, and the obstruction as seen from the window position where that is possible from the street.

Winter photographs are worth taking deliberately. A summer photograph of a facade behind a mature street tree tells a reader nothing about the sky visible in February, and the opposite mistake is equally common.

Where point clouds and drone surveys stop being useful

Terrestrial laser scanning is the best external record available and it has two limits. It cannot see through glass into a room, so it tells you nothing about room depth or use. And photogrammetric meshes derived from scan or photo data smooth window reveals, which understates the reveal depth unless you measure that dimension separately.

Drone survey adds elevation coverage on tall or inaccessible facades. It also brings operator competence requirements, airspace restrictions and overflight considerations that have to be dealt with before the flight rather than explained afterwards, and it produces the same blind spot on internal geometry.

Neither technology removes the need for the assumptions section. They make the external half of the schedule defensible, which is worth paying for, and they leave the contested half exactly where it was.

Stating the limitations so the report survives challenge

A limitations section is not an admission of weakness. It is the part of the report that makes the rest of it usable.

It should state which windows were internally surveyed, which were measured externally and by what method, which values were assumed and on what basis, every access request made with dates and outcomes, the sensitivity testing carried out and its results, and any window excluded from the assessment with the reason.

Two further points are worth including explicitly. Daylight and sunlight in planning is a different exercise from rights of light, which is a private law matter assessed by a different method and against a different threshold, and conflating the two in a planning report invites a correction. And local plan policies apply the national guidance in materially different ways, which is covered in our note on local plan amenity policies and BRE 209, so the criteria being tested should be named and sourced rather than assumed.

Where an objector is likely to produce internal photographs showing a room is a bedroom rather than the store you assumed, the report that already tested that scenario is in a much stronger position. Our note on objecting on daylight grounds sets out the evidence that neighbours increasingly bring, and reading them from the assessor side of the table is time well spent.

The survey schedule, the assumptions and the limitations together form the evidential base of a Daylight and Sunlight Assessment, and councils expect that work from a suitably qualified author with professional review before submission. Reviewers who sign off this kind of analysis can be approached through the professional directory.

If you want to see how the assessment fits with the rest of a scheme's evidence base, enter the postcode on PlanScope and work through the site constraints.

This is general guidance, not legal advice, so check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.