Since smoke alarms are required to receive their power from the building wiring and if there is more than one unit they must be hard wired together for new construction. (residential) Would it be okay to interlock them via radio frequency for if it is an existing installation? Assume you can't get from point A to point B without removing drywall.

I believe there is at least one UL listed system that uses wireless technology, in place of that third wire, to interlock the units. If nothing else, this makes it easier to add such detectors to existing buildings, and place them on different circuits. (Some homes are large enough to push the limits of one circuit).

Assuming the system was listed for such use, I would have no problem with it. Code only requires one tripped detector to result in all of them sounding off ... Code is silaent as to how that is accomplished.

There's an exception for remodelling work that allows you to not tie new smoke alarms into existing ones if it would require demolition. I can't quote you the code offhand, but I know I saw it.

I have used them on a few occasions. Yes they are UL listed for the application. The only problem I found with them is the price, even if its only 1 location you cannot reach you will need 2 units at about $100-$120 a piece.

For the sake of lurkers or others reading here who may be led offtrack I feel it might be wise to add here that the NEC does not address the location or requirements for smoke detectors. Some of the above posts use the word code, and I believe they mean building code when they refer to that word. The IRC seems to have more stringent requirements than the NFPA suggests in its recomendations. The Universal building code allows area's of a dwelling not touched by new remodel work to use battery type, not interlocked to the other smokes in the locations normally required in new construction to have smoke alarms. I think the IRC says that if it is possible to wire those untouched in area's of a dwelling with interlocked smoke alarms depending on the availability of attics or crawlspaces and so forth you must. I have no idea what the old Southern Standard Building Code would say on the subject, when I used to work under that one, smokes weren't even required anyplace yet.