sorry about the typos my phone wouldnt let me go back and edit

It depends on what type of lawn it is. If it spreads by stolons, like bermuda or St. Agustine, you will need to remove it or you will be pulling up runners for years. Rototilling won't help.
If it is a grass that doesn't have runners, you can cover it wil newspaper and put your soil right on top.

I grow in raised beds too. I do remove the grass, place a sheet of weedblock, and then lay a foundation of cardboard 2 layers thick. I still get 3 or 4 shoots of grass through the year but its easier to manage this way. 8" in plenty. I'm getting along quite will with 6" deep beds :)

Good luck!

LC

Plants usually need more then 8 inches for the root system. So you have two choices - dig in or raise high. When I started my garden where I live now there was no even lawn loam. It was fill - gravel, bricks and clay/sand with 2 inches of top soil on top. I couldn't afford to build high beds and buy all the soil I need for them, so I dug about a foot deep, sifted out rocks and derbies from clay/sand and mixed what left with compost and pit moss I bought. Then I built additional 10 inches wooden beds on top. I placed about 5 inches of gravel I sifted out on the bottom of my holes for drainage, then placed turf I dug from the lawn grass down, roots up and then filled the rest with the soil mix I made. Yea... That was a big job. But at the time I didn't see any other options - I was just starting the garden and didn't want to invest much in the project with unknown outcome).