My lawn is very uneven - it seems that between trees & settling fill there are places where there are dips of up to 8".
I'm going to get a load of topsoil, but I'm wondering how much dirt I can place over existing grass before I will need to re-seed the area?
Not much, for small holes you can roll your existing sod back and fill in the hole, then replant your sod, if it is in good shape. If you are doing your whole yard, probably better to reseed, depending on the age of your lawn you may be getting a more modern water-wise variety anyway.
