Following on from my questions, I am a lot clearer on things so I'd like to start a fresh thread focussed on a design.   Smoke Alarms 1 & 2 were installed (badly) before the extension (shown by the dashed line) was built. They are connected by T&E not 3&E so were never interlinked even though they should be. I'm trying to find out the requirements I need to meet i.e. Buiding Regs and not break the bank. Consumer unit is under the cupboard under the stairs and the alarms hav