I'm knocking down my PPOR, hopefully later this year, to build a duplex - one to live in, one to rent out. I've got pretty much equal parts cash (currently in PPOR offset) and equity (already financed, sitting in offset account against equity loan). The two properties will be similar, using similar land and buildings of similar size. I don't want to contaminate my equity loan by using it for PPOR purposes, but I would like to maximise the cash I have left after the build. If I, for instance, paid 45% of the pre-construction expenses (planning, drawings, 20% of hard construction costs, etc) from equity and 55% from cash, would that be a relatively safe way to ensure that I am able to deduct all the equity interest without contaminating the equity loan? I'm saying 45/55 because there could be a slight variation in the end values for the properties. Hope that makes sense.