Radial circuits are used for domestic lighting, meaning that the supply cable runs from one fitting to the next until the last where it terminates. There are two types of radial circuit used but more often than not the system in any one house is a mixture of the two.
Loop type lighting circuit
Junction box lighting circuit
Loop and junction box lighting circuits
Be sure you know the rules before starting any work.
With 1 mm sq. cable, one circuit may serve up to 1200 Watts and must not exceed 83 m of cable length measured without the switch length. However, where there are long cable lengths, use 1.5 mm sq. cable instead which allows a maximum cable length of 126 m measured without the switch length. The fuse rating for these circuits is 5 amp with ordinary fuses or 6 amp with MCBs (miniature circuit breakers). These cable lengths assume that the surrounding temperature does not exceed 30 degrees centigrade and that cables are not bunched together or covered with insulation of any kind.
Even if your circuits do not reach the maximum permissible load, it is normal to have at least a separate circuit for upstairs and downstairs. In the event of one fuse blowing, you will not be plunged into total darkness.