Wall Street's home buying spree amounts to a huge bet that the homeownership rate will stay low and rents rise; that's far less certain under the new tax code, according to Fannie Mae's chief economist Douglas Duncan.

DUNCAN: On the tax change it will probably have several influences. Let’s start on the consumer side, the increase in the standard deduction for renters will allow them to save more, or pay down other debts, and potentially become owners sooner, therefore moving out of the rental space and possibly putting some slow down on rent appreciation.

Second, the corporate tax rate reduction for builders may well see them lowering the hurdle rate for new investment in expanding supply, and that increased supply may well compete with some of the single-family rentals from a capital appreciation perspective.

And then, thirdly, if the economy grows faster, returns on other asset classes may improve relative to returns on single-family investments by institutional investors. It probably won't have a huge impact on the “moms and pops” which are the majority of the owners of single-family rentals, but for the institutional investors those are some considerations, I think.