This is also complex though; REITs are one of the biggest builders of new housing stock in the US, so you'd reduce creation of supply (and therefore put upward

This is also complex though; REITs are one of the biggest builders of new housing stock in the US, so you'd reduce creation of supply (and therefore put upward pressure on prices) if you broadly tax this sort of investment. I think you need to do some careful calculations to be sure that this nets out as a positive effect on housing prices. I think it's clearly problematic for investors to be holding properties vacant as a speculative investment, and we should definitely tax that (probably at the local level) so heavily that the neighbors are happy to have someone contributing so much while not being a drain on services. It's not clear to me what % of housing stock is actually being held back like this though. In the middle you have the house-flip; buy property, improve it, sell it for a profit. AFAIK most of the time this involves actually doing work on the property, rather than just sitting on it and waiting for the value to appreciate; in that case a real service is being provided, as many families don't want to buy a house and have to wait to improve it themselves. So again, if you don't target your proposed tax well enough, you might actually decrease supply at the mid-/high-end of the market and therefore put upwards not downwards pressure on housing prices. To be clear I'm not arguing against making any investigation into the policy you propose, just suggesting that it's more nuanced than you appear to be stating.