I'm going to take a guess that you are searching to buy a home and found this one. Six inches is quite of difference from front to back. This is no small problem. I wouldn't tolerate more than 1/2", but that's me. Another matter to consider in today's high priced home market.. after you spend somewhere in the range of $15K or more to level this one.. will the value of this house still be in the ball park with the rest of the neighborhood? Are they willing to drop their price by the amount it will cost to repair it? Where will you live while this work is going on? Since this isn't anywhere near a "do-it-yourself" project, I see three choices here.. either spend lots of money to make it right, get use to walking up hill and down or Keep searching..