You can lay turf on ‘dirt’ but we don’t recommend it.
Usually it is best to get a bobcat in and excavate the dirt and replace it with decent soil.
At least 20-4ml will give your turf a good start and it is also a lot easier to screed than dirt and rubble. If you’re on a really tight budget then you can skip this, but having seen the difference in lawns with good soil v dirt… I’d recommend you don’t.
Preparing The Ground for Turf