So far as I'm aware, there is nothing in the Building Regs. which specify a minimum number for any room.
There are some (very vague) references to sockets and switch locations in part M, however:
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Danger Will Robinson! Link is to the dreaded Office of Deputy Prime Minister site! ***
http://www.odpm.gov.uk/stellent/groups/odpm_buildreg/documents/page/odpm_breg_0254 94-06.hcsp [This message has been edited by pauluk (edited 08-09-2004).]