When we had our loft conversion done about 14 years ago one of the conditions of the building regulations was that we had a mains wired smoke alarm fitted to every floor. The house was being rewired at the same time so it wasn’t a big job to do and as they were mains units I wouldn’t have to worry about replacing the batteries every year or so.
What I didn’t know at the time is that smoke alarms have a life of about 10 years, after which the sensors lose sensitivity and so should be replaced. The reason I found this out was that two of our alarms started behaving oddly and on looking inside I could see that the built in capacitors were bursting. The units I had used a charging capacitor circuit instead of batteries for the back up system. It was when I enquired about replacing the capacitors that I was told the smoke alarms themselves were past the end of their life.
It seems that capacitor systems are no longer used in smoke alarms, presumably because of the problem I had myself experienced. If you look at websites of electrical suppliers such as Edwardes Bros, you will see that there are in fact now several different types of smoke alarms for different circumstances.
I had to have a mains powered circuit as that was what building regulations specified, but I think that I would choose that anyway for its reliability. As a back up some alarms still use Alkaline batteries but with an alkaline battery you have to remember to change the battery, ideally once a year. Better than that are the alarms that come with a Lithium battery installed. A lithium battery has a life of 10 years or so and so doesn’t need changing, fit it and forget it!
Another advantage I had is that my house was being rewired and the ceilings were being replaced because of the loft conversion. However it could be a bit of a faff to install wires for interconnecting smoke alarms if you weren’t having your ceilings done. The easy solution for that is to fit radio link alarms which do not need the interconnect wires but link together wirelessly.
But any smoke alarm is better than none so as a minimum fit a battery alarm such as a simple battery operated one for under a fiver. Ideally you should have at least one on each floor of the house.
A good solution for battery operated alarms which are wirelessly interconnected would be a couple of Lithium battery powered units which would take no longer to fit than standard battery units but with the advantage that they can be fit and then remain maintenance free for 10 years. In fact I would myself consider these as an ideal unit to install above a mains unit as it is simple to do and completely hassle free and involves no wiring at all. Anyone could install a couple of these.
In situations where regulations specify a mains wired unit have a look at this range which offer a unit for any circumstance with the benefit of the 10 year life of the back up circuit.
No-one should really be without a smoke alarm nowadays, especially as they are so easy to fit and really need no maintenance once they are in.