The London Article 4 Coverage Index

An Article 4 direction removes permitted development rights that would otherwise let a homeowner extend, convert or alter a property without planning permission. Some directions are narrow — a single conservation area's window and roof rules. Others are explicitly borough-wide: several outer London boroughs have adopted a single Article 4 direction removing permitted development for house-to-HMO conversions, or for basement works, across the entire borough.

This index measures, for each of the 33 London boroughs, the share of the borough's own area that falls inside at least one mapped Article 4 direction — computed by sampling a grid of points across each borough's real boundary (not its bounding box) against the official planning.data.gov.uk geometry, the same dataset and the same point-in-polygon method the rest of this site uses for individual address checks.

Read the caveats before citing a number: the percentage is blended across every direction type that borough has (see "What the percentage does and doesn't mean" below), and 19 of the 33 boroughs have no Article 4 geometry at all in the national dataset — that is a dataset-coverage gap, not evidence those boroughs have no Article 4 directions. Both are explained in full below and in the downloadable data.

The Article 4 percentage is blended across every direction type mapped for that borough — from narrow, conservation-area-linked design-control directions (materials, windows, roofs) to explicit borough-wide policy directions (several outer boroughs have adopted a single Article 4 direction removing permitted development for HMO conversions, or for basement works, across the WHOLE borough). A high percentage means some Article 4 restriction touches most of the borough — it does NOT mean every permitted development right is lost everywhere; the specific right removed depends on which direction(s) apply at that point. hasBlanketDirection flags boroughs where at least one single named direction's own footprint spans ~90%+ of the borough (a purely geometric signal, not a classification of what the direction restricts). directionNames lists every distinct mapped direction so the specific scope can always be checked.

Grid-sampling over each borough's own boundary polygon (not a bounding-box rectangle): coverage percent = sample points falling inside a mapped Article 4 direction (or conservation area) divided by sample points falling inside the borough. Measures MAPPED AREA-WIDE Article 4 directions only — the national dataset systematically omits property-specific Article 4 directions, so every percentage here is a floor, not a ceiling. A borough marked "not_held" has zero geometry for that dataset in the national dataset; that reflects a dataset-coverage gap, not evidence the borough has no such designations — confirm on the council's own planning register.