Are laptop USBs GND same as house GND
If the laptop isn't plugged in, then no. It would be running on battery power alone.

If the laptop is plugged in, all bets are off. It depends on whether the laptop power adapter is earthed or not. Even if the plug does have an earth connection, there is no guarantee that it is connected to the laptop/USB ground.

In any case you don't want to be relying on earth for signalling. The noise will be terrible (e.g. mains hum), there is no guarantee the socket is properly earthed, etc.

Always run a GND wire with whatever signals you are running (unless they are differential using a scheme which doesn't require a common ground).



You can add isolation through an optocoupler if your signalling scheme allows. Whether or not that is needed will depend on what is at the other end. If for example you had a circuit which controlled whether a mains socket was turned on or off, you would definitely want some isolation there.