Just happen to have a copy of the approved plans for my extension to
hand and they specify :-

"Install mechanical ventilation to bathroom & wc to extract at a min
rate of 15l/s switched with light & with min 15 minutes overrun."

The latter is the bit I know I'm going to hate when someone uses the
loo in the middle of the night and a noisy fan kicks in for 15 minutes.
Anybody know whether there is such a thing as a silent fan ? Any
noise seems loud at 3am!