The no-sky line in a daylight and sunlight assessment

The no-sky line divides the parts of a room that can see the sky from the parts that cannot. Daylight distribution, the test built around it, asks how much of the working plane still has a view of sky once a new building goes up next door. Where the area with a sky view falls to less than 0.8 times its former value, the BRE guidance on site layout planning treats the loss as noticeable to the occupants. That single ratio decides a great many neighbour amenity objections, and it is the part of a daylight and sunlight assessment that most non-specialist readers skip.

Vertical sky component measures light arriving at the window. Daylight distribution measures where that light lands inside the room. The two answer different questions, and a scheme can pass one comfortably while failing the other.

What the no-sky line measures

The test works on a horizontal working plane inside the room, taken at 0.85 m above floor level in a dwelling. A point on that plane is either able to see some part of the sky through the window, or it is not. Join up the boundary points and you have the no-sky line, a contour running roughly parallel to the window wall and bending around reveals, balconies and projections.

Two areas then get measured: the area of working plane with a sky view before the development, and the same area after. The ratio of the second to the first is what the assessment reports. Below 0.8, the guidance says the reduction will be apparent to people using the room.

What makes the test sharp is that it is a geometric calculation, not a judgement. Given the room dimensions, the window position and the obstruction, two competent assessors should produce the same number.

When distribution is tested alongside vertical sky component

In practice the two tests run together in any daylight and sunlight assessment prepared for a scheme with close neighbours. Vertical sky component is calculated first, at the centre of the outside face of each window, because it needs only the window position and the obstruction. The benchmark is 27 per cent, with the same 0.8 relative rule applied where the existing value is already below that.

Daylight distribution needs more. It needs the internal plan of the neighbouring room, the depth from window wall to back wall, the position of internal partitions and the height of the working plane. That extra information is the reason it is sometimes left out, and the reason an officer will ask for it when the rooms behind the affected windows are deep.

The order matters when you are budgeting the work. If every window comfortably retains 27 per cent vertical sky component, distribution rarely changes the answer. Where windows fall below the benchmark, or fall below 0.8 of the existing value, the distribution result is what tells you whether the loss is confined to the back of the room or reaches the sitting area.

Room use and layout assumptions that change the result

The BRE guidance applies its criteria to main habitable rooms. Living rooms, dining rooms and kitchens carry the most weight. Bedrooms matter less, and bathrooms, hallways, stores and garages are not assessed at all.

So the assessor has to decide what each affected room is. For the applicant's own building that is straightforward. For the neighbour's building it is guesswork unless floor plans exist, and the guesses move the result. A room recorded as a bedroom attracts less weight than the same room recorded as a living room, and a room assumed to be 4 m deep produces a very different no-sky line from one assumed to be 7 m deep.

Three sources usually settle it. Historic planning applications on the council's public register often contain floor plans for the neighbouring property. Estate agent particulars and property listings carry room labels and sometimes dimensions. External observation gives window heights, cill levels and the presence of rear extensions.

State every assumption in the report, room by room, and say where it came from. An assessment that reports a 0.74 ratio without saying how the room depth was derived invites an objector to produce their own plan and a different answer.

Plotting the contour and reporting the area

The output is a drawing and a table. The drawing shows the room in plan with the no-sky line in the existing condition and the no-sky line in the proposed condition, usually as two contours with the lost area hatched between them. The table gives, for each room:

• the room reference, the floor it sits on and the assumed use
• the total working plane area in m²
• the area with a sky view before development, in m²
• the area with a sky view after development, in m²
• the ratio of after to before, to two decimal places

Report the absolute areas as well as the ratio. A room that loses sky view across 0.4 m² of a small kitchen is a different proposition from one that loses 6 m² across a through lounge, even where both land on 0.78.

Where the assessment also covers loss of sunlight to gardens or amenity space, that work belongs in a separate overshadowing study rather than being folded into the daylight tables.

Reading a result below the 0.8 ratio

A ratio below 0.8 is not a refusal. It is a trigger for judgement, and the guidance itself is explicit that its numerical criteria are advisory and should be applied flexibly in dense urban settings where existing daylight levels are already constrained.

What an inspector or officer wants to see next is context. How many rooms are affected, and what are they. How far below 0.8 the ratio falls, because 0.79 and 0.45 are not the same finding. Whether the affected room has more than one window, or a second aspect, or a conservatory or deep extension of the neighbour's own making that caused the depth problem in the first place. Whether the retained area still covers the part of the room people actually use.

An illustrative example makes the point. Take a Victorian terrace where the neighbour built a 4 m rear extension some years ago, leaving a 7 m deep kitchen and dining space lit by a single rear window. A modest two storey extension next door drops the distribution ratio to 0.72. The vertical sky component at the window barely moves, because the obstruction is oblique. The honest reading is that most of the loss falls in the far third of a room already compromised by the neighbour's own extension. That is a materially different case from the same ratio in an unextended room, and it is worth two paragraphs of narrative rather than a line in a table.

Where the test catches what vertical sky component misses

Vertical sky component is a window-face measurement, so it is blind to what happens inside. Distribution picks up three situations it cannot.

Deep rooms are the first. A room 8 m deep behind a single window may retain a healthy vertical sky component while most of its floor area never sees sky at all, before or after.

Windows under balconies, overhangs and recessed reveals are the second. The overhang cuts the view of sky from the back of the room disproportionately, and the no-sky line moves forward sharply.

Partial obstruction of a wide window is the third. Where a new building blocks one end of a wide opening, the window-centre measurement may barely register the change while a sizeable wedge of the room loses its sky view.

Presenting the results so an officer can check them

Give the officer what they need to reproduce the finding without calling a consultant. That means a site plan showing the existing and proposed massing with dimensions, the source of the neighbouring floor plans, the assumed room uses, the working plane height used, and the software or method applied. State the edition of the BRE guidance you worked to, because the criteria and the emphasis changed between editions and older reports are still in circulation.

Where you have agreed the room assumptions with the case officer or with the neighbour in advance, say so. Where you have not been able to obtain internal information and have assumed a worst case, say that too and show the sensitivity: what the ratio becomes at the shallower and deeper assumed depths.

If you are building the wider document set around the amenity evidence, the full index of report types shows which of them a scheme of this kind usually triggers, and entering the postcode on PlanScope will flag the constraints attached to the site. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Daylight modelling itself needs a suitably qualified assessor, and a draft is a starting point for that person rather than a substitute for the calculation.