This has been a requirement in rented houses and flats in the UK recently.
For those thinking of adding these in a private house, AICO linked optical detectors work well and reliable and have a 10 year life. They are mains connected and use radio frequency to link up the detectors.
In a normal smallish house, battery independent alarms are fine for warning of smoke.
I have 8 in my house but they do need battery changing at about 2 years. Avoid the fire angel brand as they seem to get false alarms and sometimes begin making crackling sounds. Fit at least 3 in the hall and stair area, 1 in each bedroom and 1 in any rooms downstairs. Heat detector in the kitchen as smoke detector would operate every time you cook.
AICO alarms are good quality units and the family firm uses a lot of these linked types