I was under the impression that by flat tax you meant flat income tax. Property taxes are, at least in California, a very local form of taxes that finance local school districts. Using property taxes for something else (ie, to provide UBI for anyone not living in the direct school district) would be a large change in how those funds are distributed.

The exact mechanism of taxation is an implementation detail. All that's need to qualify as "flat" is that the tax paid must be in fixed proportion to the property or activity being taxed. The only reason I suggested other taxes is because precisely defining "income" in such a way that taxpayers can't manipulate it to their advantage is difficult.

Oh so all we need to do is eliminate the principle of Federalism and govern the entire country as one large state then?