Built-up areas were introduced by the UK’s Office for National Statistics (ONS) for reporting the results of the 2011 Census. The statistics collected are easily correlated with administrative boundaries such as wards and boroughs, but they do not always correspond to the traditional or historical villages, towns and cities which people associate with where they live. Such settlements have no administrative boundaries, and therefore no well-defined extent, so the ONS introduced the category of built-up area as being more identifiable with those historical settlements.
The ONS describe their definition of a built-up area as following a “bricks and mortar” approach. In essence, each area is defined as built-up land with a minimum area of 20 hectares (49 acres), and combining settlements within 200 metres (656 ft) of each other into one. The name then given to the area is that of the part with the largest area.