Light Switch Repeater

What will happen to the switch in the garage - do you want that also to switch both lights, or will you do away with it? Having two-way switching across two different circuits is a bit fraught. Maybe a relay or two somewhere?

I don't really mind if the switch in the garage just gets left on (and then the light is switched by some other device) or gets replaced by whatever the other device is.

The Philips Hue system will do that job but not by sensing the voltage, by being able to use kinetic switches which never need a battery or any wiring and can be placed anywhere and you program it to do anything you want lights wise when the switch is used. Easy to add to the current system just for those two lights.

I can do that with the Quinetic system as indicated in the orginal post but since we already have the light switch and it is one of a block of four it would be nice to be able to retain it.

If you want to do it without any new wires, then ideally you would need something like the Qinetic receiver, but some way of replacing the switch with something that can sense a mains voltage.

I would would be relatively easy with a couple of Arduino nanos - a pair of nRF24L01 radio modules, then a ZMPT101B voltage sense module on one, and a relay on the other end.

Perhaps I haven't fully understood, but ... I presume the issue is not wanting to damage the wall (otherwise you could add switches, split the existing 4-switch into 2 or 3 doubles, etcetera) so could you not trace the cables back to a convenient place (in the floor void or roof space) and add a relay? If wiring from relay is an issue then use it to operate something like this

Could you fit a swich for the whole garage supply then modify the garage light switch make it permanently in 'on' position? Then when you want the light on you can switch the garage on. When you want the light off, switch the garage off. It would mean when the garage supply is needed during daylight hours the light would be on unnecessarily but you would be talking about 4 Watts or something. The cost of the extra electricity would be far, far less than the electronics needed to do in in any other way.

Alternatively, what about wiring the garage light to a socket in the garage fitted with a remote control adapter? Would the range of the controller extend to the garage?

I could do any of that easily with the Quinetic system with a switch in the house. What I cannot find is a way of doing it slaved of the existing switch in the house instead of having an additional one.

That would certainly do it. There is not room behind the existing light switch but it could be fitted in a suitable place in the wiring under the floorboards. But the transmitter requires batteries and I don't fancy having to lift the floorboards each time the batteries need replacing. Why, when it is connected to the mains is it battery powered?

Is there a power point in the garage or just the light?

There is a lighting circuit, a 13A ring and a 16A circuit. So no shortage of power.