YEP - Definately spend the time designing and building a frasme/way to raise the coop!! My first coop was raised, but not enough. The rats moved in and by burrowing raised the soil level by 6". the cats couldnt get in and i had a hell of a trouble getting rid of the rats. They chewed their way into the coop several times...
We moved the whole coop to a new field last year and I also raised it up to 18" above ground level. - No Rats now!! the chooks can dustbath underneath it, too. i am much happier knowing that there are no rats there.
Emma T