What is underneath matters more than what is on top

The part of the job you will never see is the part that decides whether it is still level in ten years.

The build-up

A patio or drive is a structure. Below the paving there should be a compacted sub-base of crushed stone, laid and compacted in layers rather than all at once, over a properly excavated and prepared formation. The paving sits on a bedding layer on top of that.

Depths depend on use and ground conditions. A pedestrian patio needs less than a driveway carrying a car, and soft or clay ground needs more than firm ground. What does not vary is that the stone has to be compacted properly, in layers.

What going wrong looks like

Paving laid straight onto soil, sand, or an existing surface without excavation looks identical on the day it is finished. It fails within a few seasons: slabs rock, levels dip where loads concentrate, joints open and weeds establish in them. By then the cost of putting it right is the whole job again, including taking up paving that is often still perfectly good.

Edges

Paving needs restraint at the edges or it migrates outwards, particularly block paving. Edge courses are usually haunched in concrete. An unrestrained edge is a common early failure and it is visible from the start if you know to look.

Comparing quotes properly

Ask what excavation depth is allowed for, what sub-base material and depth, whether it is compacted in layers, how the paving is bedded and how edges are restrained. Two quotes with a large gap between them usually differ here, and this is the part you cannot inspect once it is finished.