An alarm that sounds only where the smoke is will not wake somebody two floors up with the doors closed, which is the exact situation domestic fires kill people in. Interlinking is what turns a set of separate alarms into a system: one detects, all sound. It is the single most valuable upgrade in most homes and it is cheaper than people expect.
What interlinking does
When any alarm in the set detects smoke or heat, every alarm in the property sounds. The person furthest from the fire, behind the most doors, gets the same warning as the person standing next to it. Since closed doors are what buy escape time, and closed doors are also what stop sound, interlinking is not a refinement: it is what makes the rest of the protection work.
Radio against cable
Cable linked alarms are wired together during installation or a rewire and are the natural choice when the wiring is accessible. Radio linked alarms pair to each other and need no cable between them, which makes retrofitting an occupied home straightforward. Both are recognised methods. Radio units still need a mains supply where the specification calls for mains powered alarms, because the link and the power are separate questions.
The Scottish standard
Scotland moved to a single standard for every home, owned and rented alike, requiring interlinked alarms: smoke alarms in the main living room and circulation spaces on each storey, a heat alarm in the kitchen, all interlinked, plus carbon monoxide alarms where there is a fuel burning appliance. If you are reading advice written for England and your property is in Scotland, this is the difference that matters.
Retrofitting a set
Mixing brands generally does not work, because the radio protocols are proprietary. Plan the whole set as one purchase, count the storeys and rooms first, and check whether the property needs mains powered units. Replacing three alarms with three interlinked ones is a morning's work and it changes the protection more than adding a fourth unlinked one ever would.
Questions people ask about linked fire alarms
Do smoke alarms have to be interlinked?
In Scotland, yes, for every home under the standard that applies to owned and rented property alike. In England the statutory minimum for rented homes does not require it, but the design standard and many licence conditions do.
Can I mix brands of interlinked alarms?
Usually not. Radio interlink protocols are proprietary, so a set needs to be from one compatible range. Plan the whole set together rather than adding to it later.
Is radio interlinking as good as wired?
Both are recognised. Radio suits retrofit in an occupied home; wired suits a rewire or new build. The link method is separate from whether the alarms must be mains powered.