I think the assertion that capping the tax deduction for home mortgages would cause the real-estate market to “crater” is in need of demonstration. As for keeping the AMT as a flat tax: Most flat taxers believe that a very important goal of tax reform is to reduce taxation on saving and investment so that it’s taxed neutrally. The AMT is worse than the regular income tax code in this respect (as it also is with respect to investments in children).