Nine inches deep is pretty minimal unless your interested in only shallow root crops and herbs. Salad greens, radishes, onions and th elike will do well, but if you wish to grow tomatos or especially potatoes, 12 to 16 inches or more is better.
As to your lawn grass... it will die out, but it will also present a problem if there are any weeds in it. Rhizomes and other creeping types of weeds will come up through your garden. Try this: see if you can either buy or borrow what we here in the U.S. call a potato fork. It's a pitchfork looking garden tool that has 4 or 5 times instead of the finer forks used for straw and hay. Use the fork to penetrate the lawn several times all along the length and breadth.. Step on it to insert the tines at least 4 or 5 inches. Then, having saved a goodly supply of newspapers, cover the entire lawn inside of your raised garden with several overlapping sheets... maybe 5 or 6. If it's windy soak them in a bucket of water first or just spray them with a lanw hose.
Then cover that with an inch or so of good organic materal like mixed loam and manure. You can buy sacks of irradiated manure at your garden store which kills the weed seeds.
Finally dump good soll in the structure. Mound it up in the middle since it will settle. In fact in succeeding years you'll have to add more soil.
We've built several over the years and really like them. You'll find that a width of about 4 feet works well. Ours are 20 feet long and are made of treated 2 X 8 lumber, one atop the other giving 16 inch depth. Don't use discarded railway ties... creosote is bad stuff. Lastly, be sure the treated lumber you use is not arsenic based...
Best of luck!