I tried that a few years ago and had two problems. 1. in my climate (zone 8) the grass never really composted and 2. the grass continues to spread. My neighbor tried covering hers with some cardboard and new soil last year and that didn't work either. If they are planning to replace the lawn with a garden bed, I would recommend looking at the articles on GW on lasagna gardening. I suspect that if you dug it up, flipped it over and built a lasagna bed on top, you would have a fighting chance.

Cheryl

I don't think there's much doubt that it WILL reappear.....if you don't put at least 8inches of soil on top.
You could spray with round up before you dig and insure the grass dies....and not harm your future plants.
Linda C

I'd remove the sod alltogether- most lawn grasses are insidious things.

you can take it out in strips 12-18" wide and the length of the yard- they make a special 'sodding knife' but you can use an edger (flat square blade on a pole) and with one person 'under' the sod mat cutting the mass away from the dirt, and another person pulling on the mat- on a good day, it comes up like carpet.

Worked for me
I use a hoe to cut into the soil and just under the roots. Then I can roll up big chunks of grass like a carpet (when grass and soil is very wet this works like a charm).

Two years ago I piled these big chunks upside down to form a long berm, covered it with sheets of cardboard and soil (to hold down the cardboard), trampled around on it for a while and left it for a year (I planted nasturtiums on it to hide it). Some grasses came through and in the fall I piled on more soil and any leftover compost and leaves. This berm has now formed the base for a raised bed.

Linda is right, you do need to cover the sod up to keep the grass from coming up again.

I've also taken the chunks of sod and just piled them in a deeply shaded corner of the house. Although some weeds grew on the surface, two seasons later I had a large pile of very good soil, full of worms and no grass.

C.