Russell Baron
Hi Tasleem You should have both smoke detectors and possibly a heat detector for your kitchen area. If you install extra wireless ones they prob wont be compatible with your existing. As Andrew says below you need mains powered ,interlinked and with battery back up in case of mains failure to comply with current building regs. Prob easier to change the 2 you already have have and just link up any extras you need. You would need to get the wiring checked on your existing to make sure you have a link cable Thanks