The previous owner of my home had approximately 300 square feet of the front yard allotted to a large flower and shrub garden and I wanted to convert it back to emerald zoiysia (sp?) grass. I learned the hard way that you absolutely need to remove the existing top soil in the area being replaced and add new fresh top soil. This is for a few reasons, but primarily it's because your new grass will slowly begin to sprout flowers and all sorts of weeds that were previously in the areas. Yes, the SOD will look great for a season, but then you'll wish you had spent an extra 3-4 hours replacing that seed contaminated soil!! Lesson learned! Replace the top soil and save yourself the pain, time, and money of ongoing issues. 😉