Smoke alarms are the cheapest, simplest life-safety device in your home, and they're also one of the most commonly overlooked items until an inspection turns up a problem.
California requires interconnected smoke alarms in every bedroom, outside each sleeping area, and on every level of the home, including basements. "Interconnected" means when one sounds, they all sound, whether that's hardwired or newer wireless-linked units.
Alarms have a shelf life. Most manufacturers rate them for 10 years from the manufacture date, printed on the back of the unit, not the install date. Past that, the sensor degrades even if the alarm still beeps during a test.
During a home inspection, we test each unit, check placement against code (not too close to kitchens or bathrooms, where false alarms are common), and verify interconnection. It's a small line item on the report, but it's one of the easiest fixes to knock out before move-in.