Worked example: sun on ground assessment on 21 March

The sun on ground assessment worked example below tests a rear garden against the two benchmarks in the Building Research Establishment guidance on site layout planning for daylight and sunlight: at least half the amenity area should receive at least two hours of sunlight on 21 March, and if the area meeting that test is reduced to less than 0.8 times its former value, the loss will be noticeable. Both limbs matter. A garden can pass the first and fail the second, and the second is where most objections are won. All figures here are illustrative.

The two hour benchmark and the area proportion

The test is about duration, not intensity. For each point in the amenity area you need the total hours of direct sunlight on 21 March, and the threshold is two hours. Those two hours do not have to be continuous, which is the detail most often got wrong in assessments of enclosed courtyards where a building casts a shadow through the middle of the day.

The spring equinox is used because it sits between the extremes. Sun positions on 21 March approximate the annual average, so a garden that works then works for most of the year, and the date gives a single reproducible basis for comparing before and after.

Two separate questions follow. The absolute question is whether at least 50 per cent of the area meets the two hour threshold. The relative question is whether the compliant area after development is at least 0.8 times the area before. Report both, with the arithmetic shown, because a council reading only one of them will reach the wrong conclusion.

Setting up the shadow plot

The model has to be right before the numbers mean anything.

1. Build the existing situation, including the proposed site's current buildings, all surrounding buildings that could cast shadow, boundary walls and fences at their measured heights, and the ground plane at its actual levels.
2. Set the location to the site's latitude and longitude rather than a regional default, because solar altitude changes with latitude and a 2 degree error moves the shadow line.
3. Set the date to 21 March and the time base to Greenwich Mean Time. British Summer Time generally begins after that date, so an assessment run on local clock time will be an hour out.
4. Define the amenity area as a polygon and record its area in m². This is a judgement: the usable garden, excluding the side passage, the bin store and anything under 1.5m wide.
5. Decide how trees are treated and state the decision. Deciduous trees in leaf are not present on 21 March, so they are usually excluded, and excluding them is the conservative choice when assessing loss.
6. Run shadow outlines at hourly intervals.

The surrounding context is where assessments are weakest. A model that stops at the red line will overstate sunlight on a site hemmed in by a three storey terrace to the south, and an objector with the same software will find it.

A worked hourly overlay from 9am to 3pm

Take an illustrative rear garden of 10m by 8m, so 80m², bounded by a 1.8m fence on each side and open to the south. A two storey rear extension is proposed on the neighbouring plot to the south west.

Plot the shadow outline at 9am, 10am, 11am, noon, 1pm, 2pm and 3pm, giving seven outlines. On 21 March the sun rises close to due east and sets close to due west, and the solar azimuth moves roughly 15 degrees per hour, so the shadow sweeps from west through north to east across the morning and afternoon.

For each hourly step, mark the part of the garden in shadow. Then count, for each sub-area, how many of the seven intervals it is sunlit. The hourly method is an approximation of continuous duration, and for a planning submission it is accepted provided you say that is what you have done. Where the margin is tight, run the calculation at 15 minute intervals instead and say so.

Counting at hourly steps tends to flatter a scheme slightly, because a point sunlit at 10am and 11am is credited with two hours even if it loses the sun at 10.20. That is a reason to use the finer interval on a marginal case rather than a reason to abandon the method.

Calculating the sunlit proportion before and after

Set the result out as a table. The arithmetic is the easy part and the presentation is what gets it accepted.

On these illustrative figures the garden just meets the 50 per cent absolute test. It fails the relative test, because 40 divided by 60 is 0.67 and the guidance treats anything below 0.8 as a noticeable loss. The conclusion to write is that the amenity area remains above the absolute benchmark but experiences a noticeable reduction in sunlight, with the extent quantified.

Had the existing figure been 48m², the ratio would have been 40 divided by 48, or 0.83, and the scheme would pass both limbs with a smaller absolute area of sunlight. That is counter-intuitive and it is exactly why the baseline has to be modelled as carefully as the proposal. Setting the baseline properly is its own exercise, and our note on setting a mirror image baseline in a daylight assessment deals with the related question for windows.

The proportional loss limb of the guidance

The 0.8 factor is the limb that decides contested cases, and it has a practical consequence for design. A garden that is already poorly sunlit has little to lose in absolute terms and a lot to lose proportionally, so the relative test bites hardest on the worst existing conditions.

That cuts both ways in negotiation. Where a baseline garden achieves 90 per cent, a scheme can take a fifth of its sunlit area and still comply. Where the baseline is 55 per cent, almost any encroachment fails. Run the baseline first and find out which situation you are in before fixing the massing.

The guidance is advisory, not a standard, and it says so. Councils weigh it alongside the character of the area and the pattern of existing development, and a dense urban context is a reason for flexibility that the numbers alone do not capture. Say that in the report rather than leaving the officer to infer it.

Sloping and enclosed amenity areas

Model the ground as it is. A garden rising to the south catches sun that a level plot would lose, and a garden falling to the south loses sun the plan view suggests it keeps. Projecting shadows onto a flat plane at a single datum on a sloping site produces an answer that is wrong in a predictable direction.

Enclosed areas need care with the boundary structures. A 1.8m close-boarded fence on the southern boundary of a small courtyard can be the controlling obstruction, not the new building, and the assessment should test that by running a scenario with the fence alone.

Balconies, terraces and communal podiums are amenity areas too, and the same test applies at their level rather than at ground level. Where a podium sits above a car park, the datum is the podium surface.

Reporting the result with the assumptions

An assessment that an officer can check is an assessment that gets accepted. Record the software and version, the latitude and longitude used, the date and the time base, the interval between shadow plots, the extent of the surrounding model and its source, the treatment of trees and boundary structures, the definition and area of the amenity area with a drawing, the existing and proposed compliant areas, both test results, and the conclusion in plain words.

Include the shadow plots themselves as a drawing series rather than only the summary table. A reader who can see the 10am and 2pm outlines will accept the number in between.

A Daylight and Sunlight Assessment usually carries this test alongside the window-based measures, and where overshadowing of neighbouring gardens is the main issue an Overshadowing Study is the narrower document. Councils expect a suitably qualified author for either and professional review is strongly advised. Keep the drawing references consistent with the rest of the submission, which is easier if the title blocks are set up properly from the start, as covered in drawing schedules and title blocks that survive validation.

PlanScope drafts the assessment structure, the policy framework and the assumptions schedule, marking missing site data as "TO BE CONFIRMED", so the modelling itself still comes from a daylight consultant. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.