[4] The boundaries are not fixed; they expand as transport options improve and affordable housing moves further away from the city centre.

Conversely it includes areas beyond Greater London such as Rickmansworth, Broxbourne, Grays, Dartford, Gravesend and Epsom.

Larger Urban Zone is a definition created in 2004 by Eurostat that measures the population and expanse of metropolitan areas across European countries.

Several large conurbations fall just outside the zone: Reading, Luton, High Wycombe and significant parts of the Aldershot and Crawley Urban Areas.

[13][14][15] (2011 Census)[16] Some estate agents, including James Pendleton[17] and Savills,[18] have defined a 'second commuter belt' further away from London.