I want to grow some grass from seed on a piece of ground at 14ft x 12ft. How deep should top soil be to grow seed on?.
You can grow grass seed in a seed tray which is only about an inch and a half deep so not a lot is the answer. However if you want to grow a lawn then I would think you need at least 6 inches of soil, properly prepared. You're not thinking of putting soil over a concrete base and sowing on that are you? because you don't particularly need 'top soil' to grow grass in. As long as it has not got extreme conditions (ie waterlogged, heavily compacted etc) you should be able to prepare any ground and sow and it should be fine.
No i've just dug up the old weed ridden lawn if you could call it a lawn, dug down 2-3 inches and compacted nice & flat. So 2-3 inches of top soil will do?.
You need to dig it over at least down a spade depth, then rake it, tread it, and rake it again. 2-3 inches is not going to do it especially if the soil underneath it is compacted.