u/Dense-Inspector-135

We are converting existing workshop to Granny, in process of getting DA done
Main challenge is to run pipes for gas, water, sewer, stormwater to/from main house to existing workshop which involves excavating, removing pavers, running pipes through retaining walls as shown in photo black lines.

And then easy is to internal fit outs, electrical work and small/easy work

Is it good to find builder who does all ? Or getting plumbing work separately and rest through builder ?
I am in Newcastle, NSW (not owner builder)
So far have discussed some builders and they are reluctant to do plumbing part rest is easy
Agree depends in many thing but any idea if it can be done and how much plumbing will cost ?

Thank you