I don't know about policy consequences, but one political hurdle is that this would be attacked as "welfare for the rich" or something along those lines, since new construction is generally more expensive than existing units.
Yeah, I haven't thought everything through yet. However, the hope that this would force more of the new housing to be more affordable. A poor person isn't going to be buying a luxury condo so I was trying to think of a way to incentivize builders to increase stock and create more affordable housing. There are definitely some second order effects.