You may be overstating if you do. A better way may be a % of cost. The GF has no land. But this may limit deductions too. You may be surprised how small a GF will comprise in area v's a family home. ie 50m2 /400 = 12.5%. So council rate deduction each year may come to little ie $150.
ie ...
Due to its cost it will not be a separate CGT asset. Special rules determine if a asset can be separated. The ATO publish this cost annually (indexed). 2014 = $138K
https://www.ato.gov.au/General/Capital-gains-tax/In-detail/Real-estate/Separate-assets-for-CGT-purposes/
Its a bit like...
