The affordable rule for a mortgage is a guideline lenders use to determine how much you can afford to borrow for a mortgage. It's generally based on a percentage of your gross monthly income that can be used towards housing costs, including mortgage payments, property taxes, heating expenses, and in some cases, condominium fees. This rule is the same income percent rule explained above. The two main ratios are the Gross Debt Service (GDS) Ratio and Total Debt Service (TDS) Ratio.