Glad people are thinking about things like this, but it has some of the same problems as previous discussion about Georgian land taxation - separating the house from the land is not actually possible.
The final recourse for a mortgage lender is repossession. That's the only reason the rates are lower than unsecured credit in the first place. If the house is separated in ownership from the land, it makes one or both portions unusable (depending on what liens/easements one has over the other). If they're NOT separable, and are always transferred (purchased or taken for nonpayment) together, then requiring cash for the land is just a very large downpayment requirement on the house+land.
Aside: I agree that 50 year mortgages probably don't solve any problems, and should probably be privately arranged (not gov't backed) in the cases that they do. Actually, almost zero mortgages should involve the feds, so it's irksome that this is even being discussed.
Epistemic Status: haven't worked through all the consequences here.
There's currently a big brouhaha going on in the interwebs about Trump's plan to allow 50 year mortgages.
Supporters claim that it'll help people buy homes.
Detractors point out that a 50 year mortgage doesn't actually reduce monthly payments that much compared to a 30 year one (because most of your payment at current rates is interest), and that insofar as it does, that just pushes house prices up since housing supply is mostly fixed in the short term (and long term is constrained more by regulation than the market).
Focusing on the second argument, the logical conclusion here is to ban mortgages entirely, since all making it easier to buy houses does is subsidise demand.
But that doesn't work either. Housing takes a certain amount to build, and if people can't afford that without mortgages, that's going to dry up or reduce new supply of houses.
It seems like the obvious solution is to allow getting a mortgage on the value of the house (even with no downpayment, even amortization free), but to require full payment on the value of the land. The value of the house, excluding the land, will be assessed using standard techniques as part of requesting a mortgage.
The effect is that houses in sought after areas become unaffordable, pulling down prices. Houses in rural areas become more easily affordable. Since supply of both is mostly fixed in the short term, the same number of houses end up changing hands in both areas, but urban buyers are saddled with less debt and rural buyers with more. However this all but guarantees that house prices in every area will be high enough to pay for new building.