Domestic detection is cheap relative to almost anything else in a house and the decisions that change the cost are few. What matters is knowing which of them changes the protection and which only changes the bill, because the differences run in both directions.
Power source
Sealed long life battery units cost more per device and need no electrician. Mains powered units with battery backup cost less per device and need wiring, which dominates the cost in an existing property. In new work or a rewire, mains is straightforward and is what building guidance expects; as a retrofit, sealed battery units are usually the sensible answer.
Interlinking
Radio interlinked units cost more than unlinked ones and this is the increment that buys the most protection, because it is what gets the warning to people asleep on another floor. If a budget has to be cut somewhere, it should not be cut here. Wired interlinking costs nothing extra in the devices and a great deal in the cabling.
Device type and count
Optical smoke alarms in circulation spaces, a heat alarm in the kitchen, and carbon monoxide alarms where there is a combustion appliance. Multi sensors cost more and are worth it where nuisance alarms have been a problem. The count follows the number of storeys and the layout rather than the floor area.
The cost nobody budgets
Replacement at end of life, roughly a decade after manufacture, for every unit at once because they were all installed together. Writing the replacement year on each unit at installation turns that from a surprise into a plan, and it is the single cheapest piece of asset management available in a house.
Questions people ask about smoke alarm cost
Is a more expensive alarm a better alarm?
Within reputable products, the meaningful differences are interlinking, sensor type and battery life rather than price. A cheap interlinked unit beats an expensive standalone one.
Do landlords have to pay for alarms?
Provision and repair duties in rented property sit with the landlord, and the specifics differ between England, Scotland, Wales and Northern Ireland.
Does an electrician have to install them?
Mains powered units need an electrician, because they are wired to a lighting circuit and interlinked by a dedicated conductor. Sealed battery units are fitted with two screws and pair over a radio link, which is why they dominate retrofit work and why a whole house can be brought up to standard in an afternoon without lifting a floorboard.