Yeah, as @BorrisTheCat mentions, in the UK it's been a requirement since the early nineties at least for houses of more than two stories, IIRC. In my place they achieved this by cabling all the smoke detectors together, but they didn't bring a mains supply to any of them! 
Instead, they went with battery operated units, all of which were completely useless by the time we moved in. They were a big name brand, but when we got in touch they hadn't produced that model for years. Nobody else made a battery-powered, wired-interconnect detector and the only one that got close was the Protect with the wireless interconnect.
I suppose I could have pulled cable from the lighting circuit, but we'd just had all the ceilings and floors done. 
They're stupidly expensive (and had some issues in the early days) but they've always worked for me. And the little pathfinder light is way more useful than I'd expected, even if it's a half acknowledgement that Google likely know my family's sleeping patterns, when the house is occupied and whether the dog needed to go out in the night.
