Is it possible to lay new turf on existing grass?

if I cover the existing grass with topsoil can i just then lay the new turf on top

I personally wouldn't do it..
If there are any weeds growing in your existing lawn, they will just poke their way through your new turf and it will just be as bad as the lawn you've just covered up..

no weeds in grass at all, just bad grass, got bold areas and not good qualtiy

we did it and it worked ok, but we dug out the deep rooted weeds first.

Why does it have bald patches? Do you have a bitch weeing on it, or is it heavy traffic from kids? You need to know so you're not wasting money on new turf for the same thing to happen.

its never really been looked after, we moved in before xmas, and the previous owners never looked after their grass, we've had it totally de-weeded, and have greenfingers come round every 3 months to feed & Weed the grass, but it still doens't look as good as fresh grass would

if it were me, I would rough up the bald patches, mix some seed with compost, and raise your mower blades.

I can skim it off first than rotovate it than new turf is 1.60 a squard meter job done