Hardwired smoke alarms have been required on new construction in the US since the 1989 NEC. It usually takes a few years for cities and states to adopt the newest NEC revision. However, by now, all states have adopted at least 1989.

Hardwired alarms are also required for major renovations. However, you basically have to take it down to the studs before this requirement is triggered.

The requirement for renovations will vary based on locally adopted codes and their interpretation by building officials. In more affluent areas, the tendency will be to require upgrades along with relatively minor work. In less affluent or rural areas, the tendency will be more libertarian - i.e. building codes and their enforcement are subject to the political process.

The middle part of my house was built in 1920 and didn't have electricity at the time, then it had uninsulated wires on insulators, then it had wiring with crumbly insulation (the natural rubber gets hard).

When we added onto the house, we put in smoke detectors (in every bedroom and hall) that are powered by the AC mains, have backup batteries and if one goes off, they all go into alarm.

If my house is ever destroyed, it will probably be an electrical fire, but my family will know about it early.

> Is hardwired power required for new construction?

Yes [0], although this is obviously country specific.

[0] http://en.wikipedia.org/wiki/Smoke_detector#Installation_and...

I don't know about everywhere, but where I live (Oregon), many of the new homes have detectors that are not only wired for power (with battery backup), but also wired together such that an alarm at one will fire all of them. The devices seem to be mostly ordinary smoke detectors except for that connection.

Have you ever taken them completely off the ceiling or just opened the battery cover? My house has 120v/9v detectors and when I was installing fire alarms in new construction, all residential multifamily and hotels had 120v smoke detectors in the units which are interconnected. They also have 9v batteries for backup. Marriott hotels actually specify 24v system smoke detectors tied into the main alarm system. I know Home Depot sells battery only detectors but those are generally for retrofit applications. A contractor will likely have to install 120v in anything new being built.

Here's a very common battery-only smoke detector: http://www.homedepot.com/p/First-Alert-Battery-Operated-Phot...

It varies by state, but in Maryland, new construction requires it, and older houses are required to install detectors with sealed-in, long-life batteries or AC detectors by 2018:

http://mgaleg.maryland.gov/2013RS/fnotes/bil_0009/sb0969.pdf