Ken:
The U.K. requirement (since 1992) is:
Detectors to be A.C. mains powered
At least one detector per story
All detectors/alarms interconnected
Detectors located no more than 7m (23 ft) from "the door of a room where a fire is likely to start," and no more than 3m (approx. 10 ft) from a bedroom door.
The requirement is part of our Building Regulations, and unlike the electrical regs. compliance is compulsory for new construction (in theory at least).
