Don't worry, I won't actually be putting in and connecting the actual wires myself, I intend to chase out channels for the wiring conduits and screw into place the switches, sockets and lamps.

Do I need a circuit breaker (Safe-T-cut) board in addition to the one I already have? The room will hardly be using up more than 3 KW at full power but the circuit breaker we have seems to be fully occupied. Will I need to earth an additional one?

Second question: my existing, fairly new safe-t-cut board does not have a test switch, should I get one installed? It is correctly earthed.

Thanks.

With no "Test" button you don't have earth leakage protection. So you'll be needing to install something at the front end.

As to your extension. If it's just another room you could simply extend your existing lighting and power circuits.

Does it have aircon and a water heater? If so you will need to add circuits for them.

Maybe a baby board in the extension fed by 6mm² from the existing board. You can almost certainly free up a breaker position by combining two lighting circuits onto one breaker, bang a 40A MCB in the now vacant slot and you're good to go.

Thanks. Ok I will buy a 40A MCB as there will be a water heater, and a test button. Looks a lot easier now.