A better angle would be "Why should we have an system that prioritizes the construction of expensive housing that literally goes to waste over the construction of affordable housing for homeless people."
In addition, get them to concede that people should earn according to their contribution, or that people "deserve the fruits of their labor." It's a bit of common sense that you can coax out of most people. The path of attack thereafter is obvious.
After all, as socialists our ultimate solution is not just to give away preexisting housing. That's charity. Eventually the population would grow and would require a new system to build more houses. The point isn't that there's more housing than homeless people, it's that capitalism results in incredible waste and inefficiency while concentrating wealth in the hands of unproductive parasites
You don't want to frame yourself as the denier of rights, you want to question the assumption that the right to private property is legitimate in the first place.
P.S. If they argue that capitalists work as investors and managers of capital, question 1) where the money came from (hint: stolen), 2) what justifies capitalists' power over workplaces (hint: If they say risk-taking and investorship a priori justifies dictatorship, pretend to agree and start arguing for monarchy, or ask them whose money is being gambled in the first place and why the rights of gambling thieves supercedes the rights of workers to earn the full value of their labor), and 3) why that's an effective way of managing an economy when it results in billions of uncirculated dollars, the sidelining of unprofitable innovations such as generic insulin, and the refusal of services such as healthcare when it's not profitable