Council say I need another smoke alarm for knocking down internal wall?

Hello, The Building Regs will make reference to the relevant BS EN 5839 whereby a detector in a newly resized room may be needed for a number of reasons (without having seen the plan of your home it could be due to means of escape, potential fire size, etc: you most likely affected the means of escape route through the corridor, and / or the potential size of the fire in the newly created room - which now presumably has two doors into the corridor). In newly built dwellings and "major refurbishments", battery only devices would not be acceptable. Your work may not seem "major" in the traditional bricks and mortar sense but the Council would argue - quite rightly - that affecting the means of safe escape in any way, is a major alteration to the safety features of your home. My recommendation to you is to purchase and install a mains operated smoke detector with battery back up, and connect it to a permanent live feed. Chances are that you can parallel the supply to the detector you already have in place (and while you are at it, the electrician should easily be able to arrange both detectors to sound an alarm in the event of one detecting smoke, using a multicore cable. Most mains detectors have this facility, or you can discard the one you have and buy two suitable units; the combined cost is very modest comparing to the works you are doing altogether). A little more work but you'll gain approval and you will be safe in the knowledge that you have a reliable means of smoke detection, especially as you have a large home and in the unlikely event of an incident, every additional second gained by an alarm clearly audible throughout the property will be precious. I hope this helps!