Yes but only a battery one, i read in 16th edition about it being connected to mains with out RCD (not sure how i can do that as my box is RCD protected) I get confused when reading regulations at times so not sure if it counts when extending the building
Was a little worried for a second there, urm i don't know about the regs for an extension but i'm sure one of the more knowledgeable guys will be along shortly to tell you.
Building regs normall require mains powered smoke alarms to be fitted to at least the hall and landing.
As for how many and where, have a look on your plans. They should look like a circle with opposite quadrants filled and usually 'SM', 'OPT' or 'HT' near it to define the type to be fitted.
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