A garden plot subdivided for one house: the walkthrough

A garden subdivision one dwelling scheme looks like the simplest application in planning and is one of the fiddliest. The principle is usually arguable. The problems are the access past the side of the existing house, a mature tree that eats the developable area, the separation distance to the rear windows of the houses behind, and a biodiversity gain plan that nobody budgeted for.

This is an illustrative walkthrough built from how these applications typically run. It is not a record of a real case, and the dimensions are illustrative.

The plot, the host dwelling and the retained garden

A 1950s detached house on a 48-metre deep plot with a 14-metre frontage, in a suburban area with no designations beyond a single protected oak near the rear boundary. The owner proposed to subdivide the rear 20 metres for one new three-bedroom detached house, served by a drive along the northern flank of the existing house.

The split left the host dwelling with about 240 m² of rear garden and gave the new plot roughly 290 m² gross, of which the house footprint took 85 m². Both figures sat above the council's supplementary guidance figure for private amenity space for a family dwelling, which is the first number an officer checks on a backland scheme.

Note the direction of travel. Applicants tend to design the new house first and then see what is left for the existing one. Officers read it the other way round, and a proposal that leaves the host dwelling with a token strip of garden is refused on the host's amenity before anyone looks at the new building.

Backland and garden land policy

Residential gardens in built-up areas are not previously developed land under the Framework, so there is no brownfield presumption to lean on. The argument has to be made on character, amenity and the ordinary development plan policies.

Most local plans carry a backland or infill policy, and they generally turn on four tests: whether the development respects the established pattern of plots and building lines, whether it causes unacceptable harm to the amenity of neighbours or the host dwelling, whether safe access can be achieved, and whether the site can accommodate the development without a cramped layout or loss of landscape features.

The character test is where local evidence wins. In this scenario the street already contained two earlier backland houses from the 1990s, both visible on the historic mapping and both with similar flank access arrangements. A short plan showing the plot pattern along both sides of the road, with the two existing backland plots marked, converted a generic objection about the established pattern into a site-specific argument the officer could address.

What does not work is arguing precedent in the abstract. Each application is determined on its own merits, and an officer who disliked the 1990s approvals is not bound by them.

Protected trees and the developable area

The oak near the rear boundary was subject to a Tree Preservation Order, which meant any work to it required consent from the authority under the tree preservation regulations, and it meant the root protection area had to be mapped before the house position was fixed.

A BS 5837:2012 survey put the stem diameter at 720 millimetres, giving a root protection area radius of around 8.6 metres on the standard twelve-times-diameter calculation, which produced a circle of nearly 230 m². The survey also assessed the tree's shade on the proposed garden and the likely future growth of the crown.

Three consequences followed:

1. The house moved forward by 3 metres to keep the whole footprint and its working space clear of the root protection area.
2. The driveway surface within the protection area was specified as a no-dig cellular construction over a geotextile, laid above existing ground level.
3. The retained garden for the new house ended up mostly under the canopy, which affected both the amenity argument and the biodiversity calculation.

An Arboricultural Impact Assessment with a tree protection plan and an arboricultural method statement went in with the application. On a plot where the tree is the constraint, do that survey before the architect starts, not after.

Access along the side of the existing house

The flank gap measured 3.6 metres between the host dwelling and the boundary fence. That is enough for a single vehicle but it generated most of the technical work.

The requirements that shaped the design:

• A usable driveway width with the boundary clear, plus a turning area on the plot so vehicles leave in forward gear onto the classified road.
• Visibility splays at the footway, with the boundary planting cut back and a condition offered to keep it below 600 millimetres.
• Fire service access. Approved Document B expects a fire appliance to reach within 45 metres of all points inside the dwelling, which on a 20-metre deep backland plot was satisfied from the street without widening the drive.
• Waste collection. Approved Document H works to containers being within 30 metres of the dwelling and no more than 25 metres from the collection point, so a bin presentation area was shown near the street frontage with a path to it.
• Noise and disturbance from vehicles passing the host dwelling's side windows, answered by relocating the drive 1 metre from the flank wall and specifying a bound surface rather than loose gravel.

The neighbour on the northern side objected to headlights, which is a real amenity point on a drive running parallel to a boundary. A 1.8-metre close-boarded fence with planting in front of it was offered and conditioned.

Overlooking and separation distances

Two sets of windows mattered: the rear windows of the host dwelling, now facing the new house, and the rear windows of the houses backing onto the site from the next street.

Most councils work to a rule of thumb of around 21 metres between directly facing habitable room windows at first floor, and around 12 metres from a habitable window to a blank flank wall or a boundary. Check the figure in the local guidance, because it varies and some authorities apply a 45-degree test instead.

In the scenario the distance to the rear elevations behind was 24 metres, which cleared the standard. The distance to the host dwelling's rear elevation was 17 metres, which did not. The design response was to put only a bathroom and a landing window in the facing elevation at first floor, both obscure-glazed and non-opening below 1.7 metres, with the bedrooms lit from the flanks. A condition removing permitted development rights for new windows in that elevation was offered and accepted, which is the trade that makes these schemes work.

Ground floor overlooking was handled with boundary treatment and a planting strip rather than with distance.

Drainage, levels and the retained boundary

The site fell about 900 millimetres from the rear boundary towards the street, which meant surface water wanted to run towards the host dwelling.

Infiltration testing to the usual soakaway method found moderately permeable sandy clay, so a soakaway was feasible but needed to sit clear of the oak's root protection area and at least 5 metres from any building. Permeable paving on the drive took the driveway run-off. The Drainage Strategy set out the hierarchy, the test results, the storage volume and the exceedance route.

Foul drainage connected to the existing private drain serving the host dwelling, which required agreement with the water undertaker and a build-over check where the new drain crossed the existing line.

Levels were the item neighbours noticed. The finished floor level was set 150 millimetres above existing ground rather than the 450 millimetres the contractor would have preferred, to keep the ridge height down and avoid a retaining wall on the southern boundary. Excavation for the foundations fell within 3 metres of the neighbouring garage, which triggered a notice under the Party Wall etc. Act 1996 on a separate timetable, the same issue that delayed the programme in a party wall notice and a rear extension.

Biodiversity net gain, the pack and the responses

The item that surprised the applicant was biodiversity net gain. A single new dwelling on garden land is not a householder application and is not automatically exempt, and the de minimis exemption only applies where the development affects less than 25 m² of on-site habitat or less than 5 metres of linear habitat. A lawn, shrub borders and a hedge comfortably exceed that.

So the scheme needed a baseline habitat survey, the statutory metric completed and a biodiversity gain plan approved by the authority after permission and before works start. The Environment Act 2021 regime expects the metric to be completed by a competent person, and a Biodiversity Net Gain Assessment drafted in advance is a starting point for that professional rather than a substitute for them. Check whether a self-build or custom build exemption applies before assuming the full process is needed.

The submitted pack was: forms and ownership certificate, location plan at 1:1250, block plan at 1:200, existing and proposed plans, elevations and sections, a street scene drawing, the arboricultural report and tree protection plan, the drainage strategy with infiltration results, the biodiversity baseline and metric, a planning statement, and a site levels survey.

Neighbour responses ran to eleven letters. Headlights and vehicle noise on the access. Loss of outlook from the properties behind. Concern about the oak. Surface water. And an objection on precedent, which the officer's report dealt with in a sentence by noting that each application is determined on its merits. Permission was granted at delegated level with conditions covering the tree protection method statement, the drainage details, the obscure glazing, the removal of permitted development rights for windows in the facing elevation, boundary treatment before occupation, and the biodiversity gain plan.

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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.