Top 10 First Time Homebuyer Mistakes! 1.Buying more house than you can afford. Typically, you don’t want your mortgage payment to be more than 28% to 34% of your monthly income. For example, if your monthly income is $5,000, a total mortgage payment of $1,500 will be in the ball park. Of course, you need to include all of your overall debt payments which should never be more than 40% to 45% of your overall bills.