The cheapest line on the quote, and one of the most important.

The problem it solves

Put crushed stone directly on clay and apply repeated loading with water present, and two things happen simultaneously. Fine clay particles pump upward into the voids in the stone, and the stone gets pressed down into the softened clay.

The interface blurs. The bottom of your base becomes a stone-and-clay mixture that has neither the drainage of the base nor the stability of compacted granular.

Nobody sees it happen. What you see, years later, is a driveway that has settled and a base that measures thinner than the day it was built.

What the fabric does

A non-woven geotextile is a permeable sheet — water passes through, particles do not. It maintains a clean boundary between the two layers so each keeps doing its job.

It also adds a modest amount of tensile reinforcement, spreading load slightly and reducing rutting on soft subgrade.

It is laid across the full excavated area with overlaps at the seams, before any granular goes in.

Why it gets skipped

Because it is invisible, it is not required by anything a homeowner is likely to check, and leaving it out saves a small amount on materials and about an hour of labour.

And because the consequence appears in year eight, not year one. By then nobody connects the settled driveway to a missing sheet of fabric.

It is the clearest example of the general rule in this trade: the parts of the job you can never inspect are the parts that decide how long it lasts.