My friends were told if they want to sell their house, in Baltimore County, Maryland, they have to bring it up to code which means wireless smoke detectors.
One person thinks this means battery-operated.
The other person thinks this means that when one goes off, they all go off. The first person thinks this might be required for new houses, but doesn't believe any place in the US requires it before selling a used house.
The house has three bedrooms, right next to each other, on the first floor, and a room in the basement that could be used as a bedroom, and a powder room in the basement. The house has a front door, a back door, and a basement door. No garage.