I have been landscaping for many years and never yet come across a job where I just start laying with no prep work, unless there is an existing, clean concrete base to lay the slabs on. Even then final height and falls come into consideration.
With no excavation, you must remember that the finished job will bring your levels up by 150mm(6"), so bear this in mind against your building as the finished job should be 150mm BELOW DPC.
Agree with above, get some quotes and advice reputable from local landscapers.