When actual costs are available, the ATO want them used. Most building contracts have ample information.
I don't see the point in paying twice the fee and sending someone to a newly built house when the build cost is available. I'm guessing they sit outside and say, 'Yep, there is definitely a house there.'
Of course, some companies will say, 'Ah, but what if there are Assets in the house that are not mentioned on the contract?'
I'm now going to make a solemn, public promise: 'If a builder ever slips into a house an additional split system air con or something like that without charging, I will eat a building contract - one of those big, fat ones with all the HIA guff.'