If once all existing floorcoverings have been removed you have a wooden subfloor, (floorboards etc) then you will need to fit plywood prior to fitting your new Karndean or Amtico floor.
In the case of concrete or some tile floors, screeding will be required to give a smooth subfloor. When you have any LVT (Karndean, Amtico, Ployfloor etc) this is vital to ensure a good finish.
Useful information- always allow screed a minimum of 3 hours to dry before you walk on it (unless it is a specialist fast drying screed). A minimum of 24 hours is required before fitting can commence, to allow enough moisture out of the floor.