Daylight and sunlight in a conservation area roof extension

Adding a storey or a mansard in a conservation area changes the daylight picture for neighbours in a way that a rear extension does not: the new mass sits at the top of the obstruction, where it has the greatest effect on the angle of visible sky from a window opposite. Run the daylight and sunlight tests on the massing before you fix the roof form, because the heritage constraints on a conservation area roof extension, setback, pitch, parapet line and materials, will close off most of the design responses you would otherwise use to fix a failure.

Permitted development will not help you here. The additional-storey rights in Schedule 2 Part 1 Class AA of the GPDO 2015 do not apply on article 2(3) land, which includes conservation areas, so this is a planning application from the start.

How added height changes the obstruction angle

The BRE guidance in Site Layout Planning for Daylight and Sunlight (2022 edition) uses a first screening test: if no part of the new development subtends an angle of more than 25 degrees to the horizontal, measured from the centre of the lowest affected window, daylight is unlikely to be materially affected. Extra height at the top of a facade pushes that angle up faster than depth at the back does.

The arithmetic is unforgiving on a narrow street. Where the facing terrace is 9 m away and the existing eaves are 8 m above the neighbour's ground floor window, the obstruction angle is already above the screening threshold, and a 2.5 m mansard adds to it. That does not mean refusal. It means the screening test is failed and the detailed tests, vertical sky component, no sky line and annual probable sunlight hours, have to be run and reported.

Neighbours across a narrow street

Facing windows across a street are the most sensitive receptors for an upward extension, and they are the ones applicants forget because the drawings focus on the host roof. The vertical sky component test compares the sky visible at the centre of the window before and after. Where the result falls below 27 per cent and to less than 0.8 times its former value, the guidance treats the reduction as noticeable.

Two things soften the picture in a historic street. First, the baseline is often already low, and where the existing value is small a modest absolute loss produces a large proportional one, which is a well understood limitation of the ratio test rather than evidence of severe harm. Second, the 2022 edition of the BRE guidance gives more room to illuminance-based assessment drawn from the European daylight standard, which can show that a room remains adequately lit even where the sky component ratio is breached. Use it as supporting analysis, and expect the officer to want the traditional figures as well.

Rear closet wings, lightwells and the back of the terrace

At the rear of a Victorian terrace the geometry is tighter. Closet wings and back additions sit close to the boundary, and a mansard on the main roof can overshadow a rear elevation window at second floor level that has no other source of light. Where the affected room is a kitchen or bathroom, the guidance is more forgiving, but the room use has to be evidenced rather than asserted from a floor plan you have never seen. Our note on room use assumptions sets out how much the result moves when that assumption changes.

Where the neighbour's window serves a habitable room lit only through a lightwell, treat the assessment as a lightwell problem, because the obstruction is dominated by the well itself.

Heritage constraints on the design responses

In a wholly unconstrained case, a daylight failure across a street is solved by setting the new storey back from the front elevation, cutting its height, or chamfering the front slope. In a conservation area each of those moves is tested against the character of the area and the statutory duty under section 72 of the Planning (Listed Buildings and Conservation Areas) Act 1990 to pay special attention to preserving or enhancing character and appearance.

That produces a real conflict. A conservation officer may want a mansard concealed behind the existing parapet with a steep front slope, matching the historic roofscape, which is the form that maximises height at the street edge. A daylight assessment may point to a setback and a shallower slope, which reads as a modern addition and weakens the heritage case. Resolving that is a design exercise, not an argument to be won by one discipline.

Options that sometimes work: keeping the mansard within the depth of the existing roof volume, taking the additional space from a rear extension rather than the front slope, reducing storey height to the minimum the floor construction allows, or setting back only at the flank where the effect on the facing windows is greatest.

Running the tests before the roof form is fixed

Test the massing first. A 20-minute study on a simple model showing the 25-degree line from the facing windows, and the no sky line contours in the most affected rooms, tells you whether you have a problem while the form is still negotiable. Our note on obstruction angles and the early massing test describes that step.

Then, once the form is settled, commission the full Daylight and Sunlight Assessment with the survey information it needs: measured window positions and sizes on the neighbouring elevations, room depths where they can be established, and an accurate existing roof profile. Assessments done from photographs and assumed dimensions get taken apart at appeal, and where sunlight to a garden or shared amenity space is in question an Overshadowing Study covers the ground the window tests do not.

Assembling the heritage and daylight evidence as one submission

The two documents have to tell the same story. A Heritage Statement that justifies a tall front slope on character grounds, submitted alongside a daylight report that recommends a setback, hands the objectors their argument.

Write the planning case so the trade-off is explicit: what the heritage-led form requires, what it costs in daylight terms, quantified, and why that balance is acceptable in this street. Officers respond much better to a stated trade-off than to two documents that ignore each other.

Where the two arguments pull apart

Sometimes there is no reconciling form. The honest response is to choose which constraint governs, evidence that choice, and accept the risk on the other. On balance, in a designated area, the heritage duty is the harder one to argue against, and applicants more often succeed by proving that the daylight loss leaves the affected rooms adequately lit in absolute terms than by winning the ratio test.

That is a judgement, and reasonable planners disagree about it.

What to submit

• Existing and proposed sections through the street and the rear, drawn to scale with the obstruction angles marked.
• Vertical sky component and no sky line results by window and room, with the baseline stated.
• Annual probable sunlight hours where a facing window is within 90 degrees of due south.
• Illuminance-based analysis where the ratio tests are breached.
• The heritage assessment, cross-referring to the daylight results by paragraph.

Draft your heritage statement in minutes on PlanScope, then commission the daylight modelling from a qualified professional and make sure both documents describe the same roof.

This is general guidance, not legal advice; 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.