If everyone had $40k/year of guaranteed income, all of a sudden opportunities to go around slumlords opens up: they can buy land and new-build or redevelop housing with UBI-Backed mortgages (so the bank knows they’re getting the money back).
Renters almost by definition aren’t capable of amassing enough capital to build new housing, so are at the mercy of the existing supply. If it is guaranteed that everyone has a capital-amassing stream of income available to them, they gain an ability to turn that stream into something longer term. Sure, this still may not be possible in areas with strict housing regulations, but a guaranteed, portable stream of income also provides people the means to move.
And why would they sell you land for less if they know you have $40k more of income? Everyone can pay that extra money, if I am a seller I am raising my prices accordingly.
Is there a reason other than naked greed, why you (philosophical 'you', not you personally) would raise your prices accordingly? Keep in mind that even as a multi-millionaire slumlord you would also receive UBI (or else it ain't universal) which is enough to cover basic food and shelter.
Because you don't directly set real estate prices based on the buyer's income. You simply sell to the highest bidder. The bidders look at their own income and decide what to pay. If all the bidders have more money, the price will tend to rise.
Renters almost by definition aren’t capable of amassing enough capital to build new housing, so are at the mercy of the existing supply. If it is guaranteed that everyone has a capital-amassing stream of income available to them, they gain an ability to turn that stream into something longer term. Sure, this still may not be possible in areas with strict housing regulations, but a guaranteed, portable stream of income also provides people the means to move.