My landlords rule of the universe is.
replace old smoke alarms 6 months ago.
more fire safety works carried out today , let me fit you another 3 smoke alarms
now I have a total of 8 in a 1 bed room flat
I don't know if I feel safe with only 2 alarms per room
some of the older ones seemed to be only powered by replace in 10 year batteries but one of them connected to the mains and they all connect to each other by wifi magic,
I guess the government is forcing mains powered smoke alarms in every room or something, since the new ones chain off the light bulb socket somehow