Single aspect flats facing a tall neighbouring building

A single aspect flat facing a tall building fails on daylight for a reason worth separating out: it is the combination of obstruction angle and room depth, not either on its own. A shallow single aspect room opposite a 10 storey block can work. A deep single aspect room opposite a four storey block often cannot. Officers and consultants who look only at the single aspect flat obstruction angle miss the half of the problem that sits inside the building.

That matters because the fix for one is nothing like the fix for the other. Reducing the obstruction is usually impossible, since it belongs to somebody else. Reducing the room depth is a design decision you control.

Why single aspect plus obstruction fails

A single aspect unit has one source of sky. Every habitable room behind that elevation depends on the same window wall, and the daylight falls away with the square of distance from it.

Put a tall obstruction in front of that wall and you cut the visible sky at the top, which is the brightest part of the sky vault. BRE 209, Site Layout Planning for Daylight and Sunlight, uses the vertical sky component to measure what is left, and a figure below 27 per cent at the window centre points to a noticeable reduction compared with an unobstructed facade.

So the window is already working with a reduced sky view. Then the room depth decides how far that reduced light penetrates. The no-sky line, which is the boundary beyond which no part of the sky is visible from working plane height, retreats towards the window as the obstruction rises. In a deep room it can end up two or three metres from the glass, leaving the back half of the space dependent on artificial lighting at midday.

Obstruction angle and its effect on the room

The quick test in BRE 209 is the angle to the top of the obstruction, measured in section from a reference point on the window. Where that angle is less than 25 degrees from the horizontal, the guidance treats skylight as likely to be adequate and no further check is needed.

Above 25 degrees you move to the vertical sky component, and above roughly 40 degrees you are usually into illuminance modelling under BS EN 17037, which BRE 209 now aligns with. The 2022 revision shifted the emphasis towards target illuminance over a proportion of the room area, which changes how a deep plan reads: a room can pass at the window and fail across the space as a whole.

Measure the angle from the actual window position, not the building line. A 600 mm reveal, a recessed balcony or a projecting floor slab above the window all cut the sky view further, and a balcony over the head of the window is the single most common cause of a failing result that the section drawing did not predict.

Depth limits for a single aspect unit

There is a published method for this. BRE guidance, following the earlier daylighting standard, gives a limiting depth relationship for a side-lit room: the room depth divided by its width, plus the depth divided by the window head height above floor level, should not exceed a value derived from the reflectance of the surfaces below window mid-height. With a typical reflectance assumption that limit works out at a value of four.

Worked the other way, the formula is a useful sketch design tool. For a room 3.6 metres wide with a window head 2.4 metres above the floor, the limiting depth comes out at around 5.2 metres. Add a significant obstruction opposite and the usable depth drops below that. This calculation is illustrative and does not replace modelling.

The practical consequence is that a single aspect unit facing a tall building wants to be wide and shallow. Deep narrow plans, which are what you get when you maximise unit count along a corridor, are exactly the wrong shape.

Reorienting units within a block

Where the obstruction is on one side only, the answer is usually plan reorganisation rather than facade tinkering. Put the circulation, stores, bathrooms and plant on the obstructed elevation. Put habitable rooms on the elevations with sky.

On a corner site this is straightforward. On a mid-terrace plot with a single open face, it is not, and the honest conclusion may be that the site supports fewer units than the appraisal assumed. Better to reach that conclusion at stage two than at committee.

Through units, with the living space at one end and bedrooms at the other, solve the problem completely but need a floorplate depth that many urban infill sites cannot give. Where the depth exists, the unit count usually falls.

Our note on daylight to single aspect flats and how to fix them goes through the remedial options in more detail.

Dual aspect ratios in local policy

Several authorities now set an expectation about the proportion of dual aspect units. The London Plan 2021 asks development to minimise the number of single aspect dwellings and says that single aspect dwellings which are north facing, have three or more bedrooms, or are exposed to high noise levels should be avoided.

North facing single aspect units in front of an obstruction are the combination to design out first. They receive no direct sun at any time of year, and the diffuse light they do receive is the light the obstruction removes.

Read the amenity policy in the adopted plan rather than assuming the national position. Our guide to local plan amenity policies and how they use BRE 209 covers the range of approaches, which runs from a bare reference to the guidance through to numeric targets with stated percentages.

Where the unit mix has to change

Sometimes the daylight answer forces a different schedule of accommodation. A family unit needs a living space and two or three bedrooms with reasonable daylight, and getting that from one obstructed elevation is hard. A studio or one bedroom unit with a single wide window is far easier.

That has viability and policy consequences, because the affordable housing mix and the family housing requirement in the local plan may pull the other way. Resolve the conflict openly in the design and access statement rather than letting the officer find it.

An illustrative example

Take an illustrative case: a six storey block on an infill plot, with a nine storey office building 14 metres away across a service road. Original design: 24 single aspect flats, rooms 7.1 metres deep, 3.0 metres wide, window head 2.3 metres. The obstruction angle at second floor level is 34 degrees. Vertical sky component at the lower floors falls below 20 per cent and illuminance modelling fails across more than half the floor area in eight units. The revision widens the units to 4.2 metres and reduces depth to 5.4 metres, moves bathrooms and stores to the obstructed corner on the lower two floors, and raises the window head to 2.6 metres. Unit count drops to 20. These figures are illustrative and not from a real scheme.

Evidence an officer will accept

Submit the section through the worst case window with the obstruction angle marked, the vertical sky component results in a table by unit and floor, and the illuminance or daylight distribution results across the room area. A Daylight and Sunlight Assessment that gives the model inputs, the reflectance assumptions, the glazing transmittance and the obstruction heights used is far harder to argue with than one that reports results alone.

State where units fall short and why that is acceptable in context: a town centre location, the balance of other benefits, the alternative designs tested. Officers accept shortfalls that are explained and quantified. They refuse shortfalls they find themselves.

Where neighbours are likely to object on their own daylight, our note on objecting on daylight grounds and the evidence that carries weight shows what the other side of the argument looks like.

Enter your postcode on PlanScope to see which reports a constrained urban site is likely to need.

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