Building a constraints appraisal from public datasets

A site constraints appraisal is a dated record of every public dataset you checked, what it showed and what it means for the scheme. It takes an afternoon for a small site, costs nothing but time, and it is the single most useful thing you can do before spending money on design or on surveys.

The point is not to produce a beautiful plan. It is to find the constraint that changes the project while changing it still costs little.

Deciding which datasets to check

Work from the constraints that stop a scheme to the ones that shape it. Flood risk, designated heritage, ecological designations and Green Belt can end a project. Trees, access and drainage usually change the design rather than the principle.

The national datasets worth checking on almost every site are these.

Add the council's own interactive map for tree preservation orders, its local plan proposals map for allocations and settlement boundaries, and the sewerage undertaker's asset records. Those three are local and the national platforms will not have them reliably.

A working order for the checks

Sequence matters, because an early answer can save you the rest.

1. Start with the flood map for planning. If the site is in Flood Zone 3b, functional floodplain, most residential proposals are finished before they start.
2. Check heritage next. A listed building on or adjoining the site, a scheduled monument, or a conservation area boundary running through it all change the consent regime rather than just the design.
3. Check the ecological designations. A site inside or close to an SSSI or an internationally designated site brings statutory consultees and a longer programme.
4. Check Green Belt and National Landscape status, because both apply policy tests that dominate everything else.
5. Then the softer layers: air quality management areas, surface water risk, agricultural land classification, priority habitat, geology and mining.
6. Finish with the local layers: tree preservation orders, allocations, Article 4 directions, parking standards and the adopted local plan policies that apply.

Step six is where most self-builders stop too early. National datasets tell you about designations. The adopted local plan tells you whether the council will support the principle at all.

Recording each source and its date

Every check needs four things recorded: the dataset name, the date you looked, what it showed for this site, and a screenshot or reference.

Do this because datasets change. Flood Zone boundaries are updated. Conservation area boundaries are extended. An Article 4 direction made six months after your appraisal changes the permitted development position entirely. A dated record tells you and your consultants what was true when the design decisions were taken.

It also matters for the application. A statement asserting the site lies in Flood Zone 1 carries weight when it names the dataset and the date. Without that, an officer has to check it themselves, and officers who have to check things tend to find other things.

Keep the raw exports as well as the summary. A biodiversity metric later on will want the habitat baseline, and our note on reading habitat inventories and classification data covers what the priority habitat layers actually mean on the ground.

Turning results into a constraints plan

Draw it. A constraints plan is the site boundary with every constraint overlaid, at a sensible scale, with a legend that names each source.

Layer order matters for legibility: the hard constraints as solid hatches, the buffer zones as dashed offsets, and the soft constraints as annotation. Root protection areas, easements along sewers, visibility splays at the access, the 21 m separation from facing habitable windows, any ancient woodland buffer. What is left in the middle is your developable area, and it is usually smaller than the site.

The discipline of drawing it is the value. A list of constraints lets you believe they all fit. A plan shows you when the root protection area and the drainage easement overlap the only place a house could go. Our companion piece on building a constraints plan before the design starts goes through the drafting conventions.

Identifying which reports the constraints trigger

Each constraint maps to evidence the council will expect.

Flood Zone 2 or 3, or a site of 1 hectare or more, points to a Flood Risk Assessment and often a sequential test. A listed building or conservation area points to a Heritage Statement and, on a listed building, to listed building consent as a separate application. Protected trees point to a BS 5837 survey. An air quality management area points to an air quality assessment. Priority habitat, ancient woodland or a designated site points to ecological survey work with seasonal windows attached.

The seasons are the part worth acting on immediately. Bat emergence surveys run from May to September, great crested newt environmental DNA sampling from mid-April to the end of June, reptile surveys through the warmer months. A constraints appraisal completed in October that identifies a probable bat roost has just told you something about next year's programme.

Browsing the full list of report types by category against your constraints list is a reasonable way to scope the pack before taking fee proposals.

Gaps that need a site visit

Public data has limits and pretending otherwise is how appraisals mislead.

Nothing online tells you the condition of the boundary fence, whether the neighbour's windows are obscure glazed, how steep the access really is, whether there is Japanese knotweed in the back corner, or whether the mature tree on the plan died two winters ago. Ground conditions are inferred from geology mapping rather than known.

Rights of way are a particular trap. The definitive map records the legal line, which is often not the worn path on the ground, and the process for checking one against the other is set out in our note on checking public rights of way on the definitive map.

Visit the site with the constraints plan printed out. Mark it up. The differences between the plan and the ground are the list of things to investigate properly.

Reviewing the appraisal before spending

Before commissioning anything, read the appraisal against three questions.

Does any constraint make the principle of development unacceptable in policy terms, rather than just difficult. Does any constraint have a seasonal or procedural lead time that dictates the programme. And is there a constraint where the data is ambiguous enough that a specialist should look before the design is committed.

Answer those honestly and the fee proposals you go out with will be for the right work. The most expensive planning mistake at this stage is not commissioning a report you did not need. It is designing for six months against a constraint you never checked.

PlanScope checks a site against flood, heritage and protected site data automatically when you enter a postcode, and flags the reports those constraints suggest. Those thresholds are the ones PlanScope uses to flag reports, and the council's own validation list and national policy take precedence. PlanScope provides AI-assisted draft reports for planning applications. 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.