How to build a site constraints plan from open data

A site constraints plan earns its place when an officer can look at one sheet and see what the site is subject to, who says so and when it was checked. Most drawings fail on the second and third of those. They show a red line, a wash of colour over part of it and no clue whether the flood layer was downloaded last week or lifted from a 2019 appraisal. Build the sheet so that every polygon on it can be traced back to a publisher and a date, and it becomes evidence rather than decoration.

Deciding which constraints belong on one sheet

Start from what the planning judgement will turn on, not from what you can download. A single sheet can carry statutory designations, physical risk layers and a short set of measured distances before it becomes unreadable. Everything else belongs in appendices or on a second sheet.

For a typical greenfield or edge-of-settlement site that usually means the development plan boundary position, flood zones, heritage designations, ecological designations and inventories, trees, and the highway and access context. On a brownfield urban plot the mix shifts towards former land use, contamination history, air quality management areas and the noise climate.

What does not belong is anything you are inferring. A polygon labelled "likely archaeological interest" mixes fact with opinion and invites an officer to treat the whole drawing as advocacy.

Keeping each layer attributable to its publisher

Every layer gets a source, and the source names the organisation rather than the portal. Conservation area and listed building extents come from the National Heritage List for England and the local authority's own mapping. Flood zones come from the Environment Agency, and for a planning submission that means the Flood Map for Planning rather than the long-term risk checker aimed at householders. Priority habitat inventories, ancient woodland and site of special scientific interest boundaries with their impact risk zones come from Natural England through the MAGIC viewer. Air quality management area boundaries come from the declaring authority, with the national register as a cross-check.

Where a national dataset and a council dataset disagree, say so on the sheet and use the council's version for anything the council will decide. That happens more often than people expect with conservation area boundaries digitised at different times.

Note the licence position too. Public datasets published under the Open Government Licence need attribution, and base mapping needs a licence of its own before it appears on a drawing you issue to a client.

Symbology that survives printing in black and white

Committee papers get printed, photocopied and scanned. A constraints plan that relies on five similar greens is useless by the time it reaches a member's reading pack.

Use hatch patterns and line weights as the primary distinction, with colour as reinforcement rather than the only signal. Keep statutory designations as solid outlines, non-statutory inventories as open hatch, and physical risk layers as tone. Label large polygons directly on the face of the drawing instead of relying on the reader to hold a legend in their head. And keep the legend on the same sheet as the drawing, at a size that survives reduction to A3.

Showing distances rather than just overlaps

Overlap is the easy case. The harder question an officer asks is how far away something is, because several policy tests turn on proximity rather than coincidence.

Dimension the nearest case for each, not the average. A tree survey will refine root protection areas later, and habitat data needs reading with the care set out in our note on habitat inventories and how they are classified, but the constraints plan should already show whether the question exists.

Dating every layer on the drawing

Put a table in the corner with one row per layer: publisher, dataset name, date downloaded, and the dataset's own currency date where it has one. Those last two differ. A habitat inventory downloaded this morning may hold survey data from a decade ago.

This table is what makes the drawing defensible a year later when a designation has changed and someone asks what you knew. It also tells whoever picks the project up after you which layers need a refresh before submission.

Separating fact from interpretation

Keep two registers on the sheet. The drawing shows what the datasets say. A short notes panel, clearly headed as commentary, is where you write that the flood zone boundary follows a ditch that has since been culverted, or that the conservation area boundary appears to follow a historic plot line rather than the current fence.

That separation protects you. If the interpretation turns out to be wrong, the drawing itself is still correct, and only the note needs amending. Mixing the two means the whole sheet loses credibility at once.

The interpretation also drives the document schedule. Flood zone coincidence points towards a Flood Risk Assessment and a sequential approach to layout, while heritage proximity points towards a Heritage Statement grounded in an assessment of significance rather than a description of the building.

Issuing the plan so it can be updated

Treat the constraints plan as a living drawing with a revision history, not a one-off output. Give it a drawing number and a revision letter, keep the source table on the face of it, and archive the raw downloads alongside the drawing file in a folder named by date.

Then set a review point. Before submission, re-download the layers that change without notice, which in practice means designations, flood mapping and anything the council maintains. Before determination, check again for anything designated since you submitted. Rights of way are worth their own check against the definitive map rather than any web viewer, as set out in our note on checking a public right of way.

As an illustrative example, a 0.4 hectare paddock on the edge of a village might sit wholly in Flood Zone 1, with the nearest Flood Zone 2 edge 40 metres away across a lane, a Grade II listed farmhouse 65 metres to the north, a priority habitat hedgerow along the eastern boundary and an unrecorded footpath shown on an old tithe map but not on the definitive map. Four of those five facts come from datasets. The fifth is interpretation, and belongs in the notes panel with a recommendation to check the definitive map statement.

PlanScope checks a site against Environment Agency flood data, Historic England listed buildings and conservation areas and Natural England designations automatically, then flags where facts are still missing, so a constraints plan and a first draft can be assembled from the same starting point. Enter your postcode on PlanScope to see which reports a site is likely to need.

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.