In the United Kingdom, the Office for National Statistics maintains a series of codes to represent a wide range of geographical areas of the UK, for use in tabulating census and other statistical data. These codes are referred to as ONS codes or GSS codes referring to the Government Statistical Service of which ONS is part.

The previous hierarchical system of codes has been replaced as from January 2011^[1] by a nine-character code for all types of geography, in which there is no relation between the code for a lower-tier area and the corresponding parent area. The ONS intends to maintain the older coding system in parallel with the new one until at least 2013.

Geography of the UK Census[link]

Information from the 2001 Census is published for a wide variety of geographical units, and a similar approach will be taken for the 2011 Census. These areas include:

• Counties in England
• Districts within English counties, and Unitary Authority areas served by one council providing district and county functions
• Unitary council areas in Wales, Scotland, and Northern Ireland
• Civil parishes (communities in Wales)
• Electoral wards (called electoral divisions in Wales). These are the areas defined for the election of local councillors, but are also widely used for presenting statistics at a smaller scale than the whole district.
• Census output areas. These are the smallest unit for which census data are published - they were initially generated to support publication of Census 2001 outputs and contain at least 40 households and 100 persons, the target size being 125 households. They were built up from postcode blocks after the census data were available, with the intention of standardising population sizes, geographical shape and social homogeneity (in terms of dwelling types and housing tenure). The output areas generated in 2001 will retained as far as possible for the publication of outputs from the 2011 census. Prior to 2001, census data were published for larger Enumeration Districts which were delineated before the census was conducted and were the organizational units for census data collection.

Neighbourhood Statistics Geography[link]

Super Output Areas (SOAs) are a set of geographical areas developed following the 2001 census, initially to facilitate the calculation of the Indices of Deprivation 2004 and subsequently for a range of additional Neighbourhood Statistics (NeSS). The aim was to produce a set of areas of consistent size, whose boundaries would not change (unlike electoral wards), suitable for the publication of data such as the Indices of Deprivation. They are an aggregation of adjacent Output Areas with similar social characteristics. Lower Layer Super Output Areas (LSOAs) typically contain 4 to 6 OAs with a population of around 1500. Middle Layer Super Output Areas (MSOAs) on average have a population of 7,200.^[2] The hierarchy of Output Areas and the two tiers of Super Output Areas have become known as the Neighbourhood Statistics Geography.

Former hierarchical coding system[link]

The older ONS code was constructed top down:

• A two-character code represented an administrative county.

For example, 12 for Cambridgeshire.

• A four-character code represented a district, so that the first two characters showed the county in which the district was placed.

For example, 12UB for Cambridge district or 12UD for Fenland.

• In the case of a unitary authority (including metropolitan and London boroughs) the first two digits were 00.

For example, 00AL for Greenwich (London Borough) or 00EC for Middlesbrough.

• Local Government wards were given a two-letter code within their local authority.

For example 12UBGA for Petersfield Ward within Cambridge district.

• The smallest level, Census Output Areas were originally given an additional 4 digits within a ward, so that the first output area in Petersfield Ward was coded 12UBGA0001.
• Civil parishes were also coded using this hierarchical system. Parishes were coded using an additional 3 digits after their local authority. For example, within 12UD for Fenland district, the parish of Tydd St. Giles was coded 12UD010.

Current GSS coding system[link]

The current system replaces these codes with a fixed length code of nine characters. The first three characters indicate the level of geography and the six digits following define the individual unit. For example the Royal Borough of Greenwich is coded as E09000011, Middlesbrough is E06000002, Cambridge E07000008 and Fenland E07000010.

The meaning of some common three character prefixes is as follows^[3]:

List of current codes[link]

The following list gives both the new style and old style codes for current local authority areas.^[4] Note that new-style codes for the Northern Ireland authority areas have not been published.^[5]