Landlords and homeowners compete between themselves for real estate near where jobs are, since there's a limited supply. This causes the market price to purchase a property to adjust to the cost of capital that is equivalent to the value gained by living there. This has a natural upper bound. The market rent is the cost of capital to purchase a property plus some margin to account for the cost of doing business. For example, if moving to an area will increase my salary by X, there's little point in me moving there if my rent will go up by more than X. The same goes for my mortgage payment. Even if I had the cash and could buy mortgage-free, it wouldn't be worth it because I could make more by investing the cash somewhere else.

If UBI is indeed universal and you'll get it regardless of where you live, then the value of "living there" will go down proportionally, because the value of living elsewhere will go up, relatively speaking. Another way of looking at this is that demand to living in expensive areas will go down because it'll be (proportionally) less necessary to live where the jobs are.