Reports for a self-build dwelling on a garden infill plot

A single self-build house on a garden plot sits at the easiest end of the infill plot planning reports question, and the document set is smaller than most first-time self-builders expect. Four or five documents will carry a straightforward application. The two that catch people out are the tree survey, because garden plots are where the mature trees are, and the biodiversity evidence, which has a self-build exemption that has to be claimed properly rather than assumed.

Start from what the council's validation list requires, then add what the site conditions force. Not the other way round.

The core documents for any new dwelling

Every application for a new dwelling needs drawings and a policy case. The drawings are a location plan at 1:1250, a block or site plan showing the whole ownership, existing and proposed floor plans and elevations, and a street scene where the plot sits in an established frontage.

The policy case is a Planning Statement. For a single house it can be short, but it has work to do that drawings cannot. It should establish the principle of development against the settlement boundary and the adopted local plan's housing policies, deal with the character of the area, and address the two points councils raise most often on garden plots: loss of garden land and the effect on the pattern of development.

Note that garden land in a built-up area is excluded from the definition of previously developed land in the NPPF. An infill plot is not brownfield, whatever the agent's particulars said, and any argument that leans on brownfield policy will be corrected.

A design and access statement is not required for a single dwelling unless the site is in a conservation area or World Heritage Site. Many applicants produce one anyway and it rarely hurts, but it is not a validation requirement outside those designations.

Trees and root protection on a mature garden

This is the survey that most often changes an infill design.

If there is any tree on or near the plot whose roots or canopy could be affected, commission a Tree Survey to BS 5837:2012 before the design is fixed. The survey categorises each tree, plots the root protection area and shows the constraint on a tree constraints plan.

For a single-stem tree the root protection area is calculated as a radius of twelve times the stem diameter measured at 1.5 m, capped at a 15 m radius. That circle is often larger than people imagine. A mature oak with a 700 mm stem produces a root protection area extending roughly 8.4 m from the trunk in every direction, which on a 12 m wide plot can rule out half the footprint.

Check whether any tree is covered by a Tree Preservation Order or sits in a conservation area, where six weeks' notice is required before works. Neighbouring trees count too, and you have no control over them.

Where the footprint has to enter a root protection area, expect a request for no-dig foundation design and an arboricultural method statement with a tree protection plan, secured by condition.

Biodiversity net gain on a small site

Mandatory biodiversity net gain applies to small sites in England from 2 April 2024, and a single new dwelling is a small site. But there are exemptions, and the one that matters here is the self-build and custom build exemption.

That exemption applies where the development consists exclusively of self-build or custom build dwellings, comprises no more than nine dwellings, and sits on a site of no more than 0.5 hectares. A single house you are building for yourself on a garden plot will usually meet all three.

The exemption is not automatic in practice. You claim it on the application form, and the council will expect the application to be consistent with it: the dwelling built by or commissioned by you, the site area within the threshold. If the plot is later sold on before the house is built, the position changes.

There is also a de minimis exemption for development affecting less than 25 m2 of onsite habitat and less than 5 m of linear habitat such as hedgerow. On a garden plot with lawn and a hedge, you will usually exceed that, so do not rely on it.

Where no exemption applies, you need a Biodiversity Net Gain Assessment using the statutory metric, and a biodiversity gain plan approved before development starts. The statutory metric is expected to be completed by a competent person under the Environment Act 2021 regime.

Protected species are a separate duty regardless of any biodiversity exemption. If there is an outbuilding with a felt roof, a mature tree with cavities, or a pond within 250 m, a Preliminary Ecological Appraisal is the sensible first step, and survey seasons will dictate the programme rather than your build schedule.

Drainage and the connection enquiries

Two separate questions: where does the surface water go, and where does the foul go.

Surface water follows the drainage hierarchy, infiltration first where ground conditions allow, then a watercourse, then a sewer as a last resort. On a garden plot a soakaway is usually the answer, and it needs an infiltration test rather than an assumption. Councils increasingly ask for the test result at application stage, not at building control.

Foul drainage normally connects to the existing house's private drain or to the public sewer. Check whether a public sewer crosses the plot before finalising the layout, because building over or near one needs the undertaker's agreement and that runs on its own timetable. A short Drainage Strategy covering both is worth producing even where the council has not asked, because it settles the question before an officer raises it.

A full flood risk assessment is only needed if the site is in Flood Zone 2 or 3, is 1 hectare or larger, or has recorded surface water risk. Most garden infill plots are in none of those.

Amenity evidence for the neighbours

The neighbours' objections are predictable and the evidence answering them costs almost nothing to produce.

Show separation distances on the site plan. Many councils use a 21 m back-to-back guideline between facing habitable room windows and 12 m or so to a blank flank, but the figures vary by authority and the local design guide governs. Show the 45-degree test in plan and section from the nearest neighbouring habitable window. Show the position of any first-floor window that faces a neighbour and whether it is obscure glazed and fixed below 1.7 m.

Where the plot is tight or the levels fall, a daylight and sunlight assessment may be worth having in reserve. Most single dwellings do not need one.

Access, parking and refuse

The highway authority is a consultee and its concerns are narrow: is the access safe, is there enough parking to avoid displacement onto the street, and can a refuse vehicle serve the property.

Show visibility splays to the local standard at the access, dimensioned. Show parking spaces at the council's adopted parking standard, with dimensions, and show where a car turns if reversing onto a classified road would otherwise be needed. Show a bin presentation point within the council's stated carry distance of the highway.

None of that needs a transport consultant on a single dwelling. It needs the drawings to answer the questions before they are asked.

What you can leave out

One more thing worth doing early. Where a Community Infrastructure Levy charging schedule is in place, a new dwelling is liable, and the self-build exemption has to be claimed and granted before you start on site, with a commencement notice served and evidence submitted after completion. Miss the sequence and the exemption is lost, which on a single house can be a five-figure mistake. A companion piece on reports for a new house on a backland garden plot covers the access and overlooking issues that a plot behind an existing house adds to this list.

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.