The requirements of sound insulation testing are fully detailed in the Building Regulations Approved Document, available here [link], under Part E (2003).
This requires all residential buildings to be tested when they involve walls and/or floors. These are known as party elements.
Assuming no tests are failed, building control bodies should stipulate at least one set of tests for every ten dwelling-houses, flats or rooms for residential purposes in a group or sub-group.
New builds must achieve impact sound test results of no more than 62dB L’nTw and airborne sound insulation of no less than 45dB DnTw+Ctr. Converted properties must achieve impact results of 64dB L’nTw or lower and airborne sound insulation of no less than 43dB DnTw+Ctr.