Building Control have said I need Smoke Detectors in my garage conversion (to a habitable room) and in the circulation areas (hall and landing). They've said separately fused to the mains board. Can I have them on the lighting circuits and is RF acceptable as they have said they must all be interlinked. They have also said that Part P is required. Is this the case if we use the lighting circuit. Any help would be appreciated.
Part P is the registering of inspection and test data with them. You should inspect and test everything that is done the sample paperwork can be downloaded from the IET web site look for theiet not allowed to give links. Or try Part P links copies of forms with that too. It is common to supply heat and smoke detectors from lighting but they must have back-up battery fitted as to linking never do this in house but in industry I would use heat prove cable and if this is required not really a DIY job.
The forms must be signed by Designer, Tester and Installer all who should be trained and unless you hold some electrical qualification you may have problems. You can get it inspected and tested but before doing any of the work yourself check what your local building inspector will accept. It also says that safety circuits must only be worked on by instructed or skilled persons 560.6.2 also 560.5 refers to supplies but the "Safety Source" will be the battery not the mains.
But more to do with building regulations than electrical so ask the building inspector what he will accept.