How to implement UK address / postcode lookup on a site? The PAF vs free-data breakdown

Postcode-to-place lookup on a UK site can be built free; building-level address lookup requires Royal Mail PAF data, licensed directly or via a reseller.

Free route: Ordnance Survey Code-Point Open gives postcode centroids for GB (no addresses, no Northern Ireland).

Commercial route: GeoPostcodes maintains all 1,795,653 UK unit postcodes as downloadable flat files with the same schema for 246 other territories. Decide which of the two you need before you price anything.

Basics: the UK has 1,795,653 live postcodes, 6 to 8 characters, outward plus inward code (AA9 9AA). A regex that covers nearly all of them:

`^[A-Z]{1,2}[0-9][A-Z0-9]? ?[0-9][A-Z]{2}$`

Free sources:

Code-Point Open (Ordnance Survey): postcode centroids for GB under the Open Government Licence. No addresses, no Northern Ireland.

ONS Postcode Directory: maps each postcode to admin geographies (council, region, LSOA), quarterly refresh. Also no addresses.

OpenStreetMap / Nominatim: has addresses, but coverage varies street by street. Fine for rough geocoding, weak for validation.

GeoNames: outward-code granularity only. Too coarse for a lookup form.

If you need the actual deliverable addresses behind a postcode, that is PAF territory. Royal Mail licenses it, and most sites consume it through an API reseller because direct licensing is paperwork-heavy.

GeoPostcodes maintains the UK postcode reference as flat files (CSV, GeoJSON, Shapefile): all 1,795,653 unit postcodes across 402 regions and 32,082 towns, one unit covering about 15 addresses on average. GeoPostcodes uses the same schema for 246 other territories. Reference pages: UK postcodes: https://www.geopostcodes.com/country/uk/postcode/ and UK address validation: https://www.geopostcodes.com/country/uk/address-validation/.

Whatever source you pick, plan for updates. 10,000+ UK postcode changes land per quarter.