Contact a quantity surveyor for the depreciation schedule.  BMT & Associates (ph92416477)have been recomeded by my accountant.  Never used their services or not sure if the phone no is correct.

Opportunity

I bought a block of land last year with the intention of building a duplex.  There is a fribro dwelling on the land which is rented after some minor renovations.  I have not read any of those books (thanks for the recomendation) nor did I know about this forum when I made the decission to buy.  Me and hubby checked with the council, their requirements on dual occuapancy and bought a piece of land that fits.
However, by looking at the land we could not decied if it was sloping towards the street or otherwise (for storm water purposes) which needed a contour survey which we did only after purchase.  In ourcase we were lucky it was sloping towards the street, if not it would have been a case of asking for an easement from the neighbour.  This is to do with our council regulations.
I am in your situation too.  Not knowing how much a duplex would cost.  But we had an idea as to approximately how much it would be.  Looks like it will cost $500,000.  Is this too much?
Is it a good idea  to go with a project builder?  They have quoted me with a sight cost of $50,000.  I think this is far too much.
By the way this is going to be in the Parrmatta council of Syndey.  Now I am wondering if we could subdevide the land and build two houses next to each other without a common wall.  I have not checked with the council the possibility for this.  I have heard that duplexed are not that much in demand.  I appreciate if anyone could advice me. 
Thanks for recomending the books,  I sure will get my hands on them.