About the voltage usage in house
All four sockets on the extension lead are wired in parallel, so they all receive the same voltage.

There will be a voltage difference between the plug that supplies the extension lead, and any of the four sockets. This is due to the current flowing in the small but finite resistance of the long lead causing a voltage drop. You can expect this drop to be insignificant in a well made lead.

This voltage drop causes power dissipation in the lead, which is why a long lead must be fully unwound before being used to power heavy loads, to allow it to dissipate this heat while staying at a safe temperature.