Bad lawn

You could spray it with something like Roundup a week or so before putting in the new soil to kill the old stuff.

Nice one, that gives me something to go at, thanks Ralph

I mentioned Roundup. For that to kill off the old stuff, it takes a week or two as far as I understand it, it does not really kill off the stuff, but prevents it from absorbing something the grass needs to grow, so it will take a while for it to be effictive. There may be some other chemicals that you may want to look into. I don't know what is out that will kill off everything fast.

...using herbicide...

better yet, go out and get cardboard for free and put it down. one or two layers with a few holes here or there for water to get through. then put the dirt on top.

songbird

Ralph Mowery wrote: ...

it takes sunshine and good growing conditions to get the weed killer down into the roots. sometimes takes more than one application.

smothering with cardboard will work better without using any poisons.

by the time the weeds and grasses underneath are dead the worms will break down the cardboard and the lawn on top will be established.

songbird

No problem....just care it by giving water regularly....you will find result soon.

He doesn't need to apply Roundup or cardboard. He said he's putting down 200mm of soil. That's 8" and existing grass, weeds, etc on the lawn aren't going to grow through that.