CGT on subdivision

How do you calculate CGT when you subdivide your block, build on the rear then sell?

i.e. what is your cost base from which profits are calculated to pay CGT?

I assume building and subdivision costs are taken as is. What about land costs, do you the value of the land when originally purchased or the value at time of subdivision?

Let's also assume it's kept for 12 months so CGT is only calculated on 50% of profit.

Thanks for any help on this one.
Last time I checked the ATO says you apportion it up from the original purchase price, less a portion of the costs, not the value at time of subdivision (which could be signficantly higher if you've left it a few years between buying and dividing).

In my case I bought a house at land value and split it half so my base for CGT is half what I paid for the original, less half what I paid to divide it. This sucks for accounting because I've ended up with a loan against the house and no loan against the new land. And sucks more because we might want to rent out the house so around half the original loan won't be tax deductable.

Not that I even have the land yet. The paperwork has vanished into the bowels of the lands and titles office, and I need the damn title to get finance to build a house on it.