What the planning system has drawn on the map
Green belt, brownfield registers, article 4 directions, conservation areas and agricultural land classification — the designations that decide what may be built in England without asking, and where land is being offered forward.
Planning constraint is the difference between a site and a proposal. The land, planning and constraint map carries sixteen layers from the MHCLG Planning Data Platform, split between what holds development back and what is being offered forward.
Restraint
The green belt is the best known and the most misunderstood: 181 parcels of it, drawn to check sprawl rather than to protect landscape quality, which is why green belt and designated landscape are different layers here and often disagree on the ground.
Alongside it, article 4 directions — 7,313 of them — do something quieter and often more decisive. An article 4 removes permitted development rights, so it is the layer that decides whether a change of use needs an application at all.
Supply
Brownfield land registers are the other half: 35,688 entries in the national register, plus a separate brownfield sites layer. These are councils publishing land they consider suitable for residential development — an offer, not a permission, and one that reflects what a local authority chose to enter.
Built-up areas and local plan boundaries give the context those sites sit in, and the NSIP layer carries the 233 nationally significant infrastructure projects that bypass the local system entirely.
The England problem
Almost everything on this map is England only. The Planning Data Platform is an English service, so brownfield registers, article 4 directions and green belt stop at the border — not because Scotland and Wales lack the concepts, but because they publish them separately and differently.
That is worth stating plainly rather than leaving to be discovered. A blank map over Cardiff is a coverage gap, not an absence of constraint, and treating one as the other is the most common way to get a confident wrong answer out of open planning data.
Sources
MHCLG Planning Data Platform — green belt, built-up areas, brownfield land registers, local plan boundaries, development corporations, article 4 directions, conservation areas, agricultural land classification and assets of community value — under the Open Government Licence.