I agree with Susan. 25% is a good number and was used in the industry for many years. I believe 33% may be a more common figure today but I lean toward caution.

My personal rule of thumb is to have monthly payments (principle,interest, taxes and insurance) not more than 25% of income. Lenders use different rules, and look at total debt. I think that buying at the limits of your loan qualification is part of the reason people buy houses that are more expensive than they can really afford. If you spend all your money on house payments, you have nothing left for all the other good things in life!