Learn more about Home Counties
"Home Counties" is a phrase used to designate the group of English counties which border or surround London. The term originated with the "Home Circuit" of the itinerant Assize Court. This was the circuit of the counties closest to London, or Westminster, where the court had its more or less permanent seat. With time the expression has lost its legal connotation, and now refers primarily to the counties in their capacity as the London Commuter Belt or the London Metropolitan Area i.e. where people can work in London without living in the city and travel "home" each evening. The Home Counties are sometimes also called London and the South-East.
If the definition is taken as those counties which border London, the scope has changed over time. In 1888, with the laying down of London's formal borders and the creation of the County of London, these prescriptively became that of Kent, Surrey, Middlesex and Essex with sections of what had been Kent, Surrey and Middlesex used to make up the new county.
In 1965, when London's borders expanded further into parts of those counties and Hertfordshire to become Greater London, the list grew to include Buckinghamshire and Hertfordshire. At this stage, the remainder of Middlesex became absorbed into London with small sections ceded to Surrey and Hertfordshire. A minor boundary change in 1995 brought about by the Berkshire, Buckinghamshire and Surrey (County Boundaries) Order 1994 gave Berkshire a border with Greater London. 
In common usage, the phrase "Home Counties" is not necessarily applied with such geographical precision, and any county in the surrounding area, within a given distance, may be considered one of the Home Counties. However, being part of the Midlands, West Country or South West England would be mutually exclusive to being one of the Home Counties, and presents a definite barrier to its scope.