Hi. Today I wanted to install an outside light. The nearest power supply was an on/off switch in the front room. I cut a new piece of cable and joined it into the switch with the existing wires. I ran the cable though to the other side of the wall into the hallway and wired them into a new switch. I then wired another length of cable into the new switch, ran the cable through the wall to the outside of the house and wired that into my outdoor light. I've obviously done something wrong as the lights are messed up. I have power going to the outside light but this seems to be controlled from the front room switch and the hallway switch and now I can't turn off the front room lights. Help please?

The nearest power supply was an on/off switch in the front room

Describe the wiring in that box and what connections you made.

Standard red black and earth in original switch. Joined new cable in same holes. New switch, Red into top hole, blacks into L1 and earths to earth connector.

Are you here in the U.S.A.

If you are, then a switch interrupts an ungrounded conductor (Hot). It does not make use of the Neutral (Return). Only recently has the electrical code be changed to require the Neutral conductor to be in the switch box outlet.

You need to go back to the original switch and remove the wiring you installed. Then find a supply branch circuit that you can splice into.

Apparently you are located in the UK or EU. I am not familiar with your wiring practices and am not comfortable advising except to say that you have taped into a switch loop and do not have a neutral. In other words it won't work properly. Consult an local electrician.

Consult an local electrician.My feelings as well.