There are two ways of extending a ring circuit, either by adding junction boxes or by using existing sockets. When extending a ring circuit, planning is essential, as much of the new part of the circuit should be installed before breaking into the existing ring circuit (allowing use of power for lights/tools while you work).
SWITCH OFF ELECTRICITY AT THE MAINS - Before carrying out any work on existing parts of the circuit switch off the mains and ensure that power has been cut before starting. You will need a length of 2.5mm2 cable to extend the ring circuit.
On the existing ring circuit to be extended, find the two sockets adjacent to the area you wish to extend the circuit into. Plan the route of the new part of the circuit based on the positions of these two points, i.e. it will start at one of the existing sockets, go round the room and end at the adjacent socket therefore preserving the ring. The length of wire between the two existing sockets can be reused between one of the existing sockets and one of the new sockets.
The digram on the left is an example of an existing ring circuit, and the diagram on the right shows the new part of the ring circuit in orange joined to the old ring circuit at an existing socket and a new socket.