db – thanks for clarifying that, and as what you’ve described falls within the scenario that I outlined in the 3rd para. of my post (#2) and as the toilet pan is on a false floor that I assume won’t be attached to the concrete floor of your garage, all you should need to do is give your Owners Corporation 14 days written notice of your proposal in accordance with Sect 116 of the NSW Strata Schemes Management Act (1996).
I say “should” because your Owners Corporation (O/C) could claim that as the sewerage line services your Lot and the other two, that it’s integral and therefore common property; unless of course there’s a physical point of connection such as a gully where your line joins the one that services the other Lots – but that’s a moot point that I doubt your O/C could sustain.
You’ll note that the preamble to Sect 116 differentiates between the structure of a Lot and its services, and that Clause 4 only refers the the former. However in my opinion, that Clause is intended to apply to both the structure and the services, that’s how I’ve always applied it and it appears from your discussions with Fair Trading and NSW L&PI that they are making the same interpretation.
So I’d go ahead and give your O/C that 14 days notice, but you’ll need to include an overview of your proposal, including hours-of-work to avoid your works causing a nuisance to the neighbours, and your tradespersons’ details as I advised before, particularity as they, their vehicles, and equipment will presumably need to be on the common property at some stage.