According to figures from the Office of National Statistics, 5% of the UK adult population have a family home and at least one other property.
Typically, second homes are a country house, but a rising number own buy-to-let investment property in town and city centres.
Those living in the south east are most likely to be part of the 2.1million holding more than one property.
7% of adults in the south east area own a second home, contrasting with only 3% in the north east, north west and Midlands.
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The most common spots for second homes are the Norfolk, Cornwall and Devon coasts. Another popular place is central London, where young couples or families with their main home in the country own pied-a-terres.
The report predicts the number of Britons owning overseas homes will jump to two million by 2025. There are two types of homeowners – pensioners who want to escape the cool British winters for warmer climes and wealthy Britons over the age of 45.
Some experts warn, however, the risk to homeowners who have money tied up in housing if property prices flatten, or even plunge.