wiring outside light to existing switch

[kekeboy]

I have a hall light operated from a single operation switch inside front door, powered by 2gang switch in passage, so enabling you to operate light from either switch. the 2gang also operates stair light independently. I am trying to connect an outside porch light to the switch in the hall so that when hall light is turned on the outside light comes on as well. Is it possible to do this using existing switch and how do i wire it please. If not poss what do i need.

[stoneyboy]

kekeboy,
You would have to run a length of twin and earth from the existing hall light to the porch light.
end

[kbrownie]

You will need to run a twin and earth cable form the hall light fitting, connecting the brown of twin and earth to the live side of light (not the loop in) and the blue to the neutral side of light of both hall way and external light.
Don't forget to install and terminate the earth cable and prove there is continuity of this conductor back to the consumer unit. The earth cable cable will require sleeving with Green&Yellow PVC sleeve.
Not sure why you want to do this though and not have separate functional switch for external light.
I personally would always recommend that outside equipment being taken off internal circuits are protected by double pole switches, to prevent total power loss of the circuit if an external fault occurs.