Hi RSP – If your side-by-side apartments are separate Lots within your Strata Plan and you're talking about physically amalgamating those, then there's a lot of work involved including valuations and the preparation and lodgement of an Application for a Strata Sub-Division with the Land & Property Management Authority (in NSW).

I won't go into the detail now just in case your apartments are not as described, but otherwise post back to the Forum for further advice.  

The last I knew, Strata Title administration in Queensland was handled by the Office of Tourism, Fair Trading, and Wine Industry Development, and Land Titles were under the Department of Environment and Resources (or similar); all that probably makes sense in QLD!

Seriously though, in NSW the bureaucracy involved in amalgamating two Lots in a Strata Scheme is exactly the same as that which applies when a Developer wants to Strata Title an new apartment block containing 100s of Lots.

You're on the right track though with engineering and design plans, and valuations, but I don't know of a one-stop-shop. Perhaps start with the Local Council who would receive similar proposals albeit on a larger scale than you propose.

Any Qld posters reading this may be better able to assist.

Not sure if this is what you are looking for in terms of advice but this might help: In most cases, there is always a 100mm stack (pipe riser) and then it branches  into a smaller pipe, it is always good to try and work your layout around this pipe.

The only tricky bit is that by law you need a floor waste in the bathroom. This is difficult because it is the only pipe you need below floor level (and in a kitchen there is no such pipe). All the others are above ground so you can work around them.

If you cannot get a pipe below floor level , what can be done is that a dry floor waste be installed (not connected to sewer). Then this gets installed close to the outside wall ,which involves coring a hole from just below finished floor level. It goes out the wall to the outside.

The only time water will discharge through this is when there has been a flood in the bathroom. When doing it this way always need to remember to install a vermin proof flap.

Give it a go, you never know your luck. But if you aren’t allowed to core to the outside, you could try this:

Forget the dry waste idea. Instead, you could create the floor waste by raising your floor level (if your ceiling height allows). You can run your waste pipe into the kitchen stack (this is an internal pipe). 

If you can imagine, you’ll be stepping up into your new bathroom and the pipework will sit under that floor.

Your waste pipe from your kitchen sink will tell you where the stack is.