having another stab in getting you to rethink of dicing up the house
instead of new GF build..
cost wise, I would think spending 20% of what you anticipate but achieving 80% rent you anticipate because of attached GF versus detached..
your plan looked similar to a job i did in blacktown last year, but your plan looks appears to be simpler hence cost should be smaller.
have a look the attached drawing.
-- fire rated wall- i don't see any difficulties. did similar. cost around $5-6k. the laundry and toilet separation doesn't have wall titles so easier.
-- see if the laundry solution works, if not there is no harm putting a external laundry plumbing outside for granny flat, and give existing laundry to front house. the plumbing might cost might be $2-3k
-- kitchen, i wouldn't think it will more than $5k including sink, range, cooktop cabinets.
--need to put a door entry for GF
-- not sure if GF tenant can walk above retaining wall
benefit is that you can always restore back to original configuration by opening the hall way. The firerated wall looks like a normal wall when finished