A UK postcode is the alphanumeric code Royal Mail uses to sort and deliver post, identifying a street, part of a street or sometimes a single large building. It has two halves separated by a space: the outward code, which gets the post to the right delivery office, and the inward code, which narrows it to a small group of addresses.
How a UK postcode works
Take SW1A 1AA as an example. It breaks down into four levels:
| Level | Example | What it covers |
|---|---|---|
| Area | SW | One or two letters, usually based on a city or town (SW is south-west London, M is Manchester) |
| District | SW1A | The area plus a number and sometimes a letter; this is the full outward code |
| Sector | SW1A 1 | The district plus the first digit of the inward code |
| Unit | SW1A 1AA | The full postcode, typically a handful of addresses |
Royal Mail keeps the master list of addresses and postcodes in the Postcode Address File (PAF), which most address lookup tools on websites draw on.
London is where people get caught out. The London postal areas, E, EC, N, NW, SE, SW, W and WC, were drawn for sorting post long before the London boroughs were created in 1965. They do not follow borough lines: N1, for instance, takes in parts of both Islington and Hackney. Much of outer London uses postcode areas named after other towns, such as BR for Bromley, CR for Croydon, HA for Harrow, KT for Kingston and RM for Romford, and some of those areas reach well beyond Greater London.
New-build developments are a common headache. Royal Mail may issue a postcode before mapping apps and address lookup tools have caught up, so customers’ sat-navs and online forms may not recognise it for months. If your premises are new, check that the pin on Google Maps sits in the right place and give clear written directions on your contact page.
Why it matters
For local marketing, postcodes are both a precise tool and a trap. When you define a service area on Google Business Profile or target ads by location, postcode districts let you draw tight boundaries. But if your location pages are organised by borough and your targeting by postcode district, they will not line up exactly, and customers do not think in either system consistently. Someone in SE9 may say they live in Eltham, Greenwich or just “south-east London”.
Postcodes also need to be written identically everywhere your address appears. A postcode missing its space, or written in lower case, is a small inconsistency, but those add up across directories and undermine NAP consistency.
Common mistakes
- Assuming postcodes match boroughs. A page about “plumbers in Islington” should not list N1 as if it were all Islington.
- Dropping the space. SW1A1AA should be SW1A 1AA, in capitals.
- A different postcode on different listings. Often the result of an office move or a typo copied between sites.
- Thin pages per postcode. Creating a near-identical page for every district reads as doorway content.
How to act on it
Check your full postcode against Royal Mail’s postcode finder and use that exact form, in capitals with a single space, on your website, Google Business Profile and every directory. In your LocalBusiness schema, put it in postalCode, with addressCountry set to GB.
When planning coverage, map which postcode districts fall in the boroughs you serve before you write location pages or set ad targeting, and decide which system each one uses. My local SEO service includes this mapping for businesses that cover several parts of London.