Craft
Drainage: Where the Water Goes After You Pave It
Paving a garden changes where its water goes. Deciding that deliberately is design; discovering it afterwards is a repair bill.
Before the work, rain landing on that part of the garden soaked in roughly where it fell. Afterwards, a large share of it runs somewhere instead. Where it runs is now a decision - whether or not anyone made it.
Two water problems, commonly confused
Surface water is what runs off the top. It is handled with falls: a slight, consistent slope carrying water away from the house and away from any wall, toward somewhere it can be absorbed or collected.
Base water is what gets through the joints, which on jointed paving is a great deal of it. That water arrives in the base and has to leave, or it sits there - and water sitting in a base through a freeze-thaw winter is the mechanism behind most heaving.
A quote that addresses only the first has addressed half the problem. This is closely tied to what is under the surface.
Falls: enough, consistent, and away
Too little fall and water ponds. Too much and furniture rocks and the surface feels wrong underfoot. What matters as much as the amount is consistency - an even fall across the whole terrace, rather than local corrections that produce puddles at the junctions.
And direction: away from the house, always. Water directed at a foundation is the most expensive mistake available in this trade.
Where it is allowed to end up
Somewhere it can soak away, into a drainage structure, or into a system that can take it - but not onto a neighbour's land, and not into a place where it simply reappears. On a shoreland parcel this stops being purely practical: the county has a view about land disturbance and runoff within 1,000 feet of the water, and directing new runoff toward the lake is not a neutral act.
The roof is part of this
Downspouts that discharged harmlessly onto grass discharge onto paving after the work, at volume, usually at the worst possible point. Extending or rerouting them is cheap during construction and irritating afterwards, and a downspout aimed at a new terrace will find its weakness within two winters.
Walls need their own answer
A retaining wall is a dam unless water can get out from behind it. Drainage aggregate, a pipe, somewhere for that pipe to discharge - and the discharge point is the part that gets omitted, which is why so many walls with visible drainage still fail. See retaining walls.
Permeable surfaces are not a free pass
Permeable paving moves the water into the ground rather than across it, which is genuinely useful. It still needs the ground beneath to accept that water, and it needs its joints maintained or it gradually stops being permeable. It is a design choice with obligations, not an exemption from thinking about drainage.
The question to ask
Point at the plan and ask where the water goes - surface water and base water, in a heavy storm, in February. A contractor who answers precisely has designed it. One who answers generally has not yet.