Depends what the ground is like really and how fussy you are boo2. I wouldn't dig anyway, as your new lawn will soon begin to sink in places. Best to hire a rotavator(about �45.00 a day) and rotavate 100-150mm deep with at least 3 or 4 passes, grade/level the area, consolidate the ground by shuffling across the whole area with your size 10's, apply some growmore fertilizer, lightly rake in and lay turf. If you are ripping up an old lawn, you would be better removing the old turf first by rotavating the top 25mm or so.