Do smoke alarms have to be interconnected?

Yes - when more than one is required, all smoke alarms must be interconnected so that triggering any one sounds them all (IRC R314.4, 2015 edition).

When a dwelling requires more than one smoke alarm, the IRC requires the alarms to be interconnected so that activation of any single alarm sounds every alarm in the unit. (This is Section R314.4 in the 2015 IRC; the section number differs in some other editions.)

In new construction the required alarms must also receive power from the building wiring with a battery backup, so they keep working during a power outage.

Limited relief can apply to existing homes during alterations or repairs where interconnection or hardwiring cannot be added without removing interior finishes.