Yes,Irene..I did the same in a border,a few years ago..it doesn't take too long to rot down as compost ,and make a decent base ...if you have too much,you can stack it somewhere upside down or add to a compost bin,for use in the future....

This works well but don't just turn the turn it over, its better to also bury it at least half a spade deep, a whole spit if you can, especially if the lawn was laid with turf cut from meadow or other coarse grass, as this has a tendency to grow through in the first season. If you have the energy to chop it a bit with the spade to fit it into the hole at the bottom that helps too. Also if you don't bury the turf deep enough you'll dig it up again every time you come to plant something.

I have done this with all my beds as we started with all lawns. It helps a bit to improve the soil and saves all that carting to the heap and carting it back when its rotted...
I have never needed to add any topsoil. After all topsoil is what you have under the lawn already.

Not a good idea to bury it -the roots will take hold again and sprout new grass. As Bloomer said, just turn it upside down until it dries out then work it into the soil. Makes a good compost.

My border was started from scratch on a new property,so the lawn in situ was dug up,and a lot of the turves were laid upside down at least a spade and a half deep,to form a base..we then stored the rest till they had rotted down..Topsoil was needed to fill the border,Paul,as it was a raised bed,and took some filling....we didn't get anything growing through,so it worked well..probably the turf used was in pretty good shape,with it being newly laid..we still prefer the gravel area in it's place :o).

I did exactly as Stera and Bloomer said and it worked a treat. Hardly anything came through.