A drain has to sit below the water it is meant to intercept and above the level its outfall can accept. For a wet yard that is usually a foot to two feet. For a foundation it is at or just below the footing. Depth is set by those two constraints together, never by a rule of thumb.

How deep the water sits here

This is the number that changes the answer in Syracuse: the shallowest mapped seasonal water table sits 3.1 inches below the surface, in ground mapped largely as Parleys loam, 0 to 4 percent slopes. That is effectively at the surface for part of the year. A trench cut to any useful depth here fills with groundwater while it is open, which affects the machine, the day and the method — and it means a drain set at the usual yard depth is sitting in the water table rather than above it. Ask what the design assumes about spring conditions specifically.

The reasoning behind that

Water travels to a drain because the drain is lower. A trench above the level the water is moving through catches only what falls into it from the surface, which is why a shallow drain in a wet yard so often disappoints.

At a foundation the target is the footing. An exterior drain set at footing level intercepts water before it reaches the wall. Set higher, it lets water through beneath it and collects what is left.

The ceiling is the outfall. A drain must fall continuously to wherever it ends, so the depth at the far end is the depth at the start plus the fall. If the outfall is shallow, the start cannot be deep, and the honest answer becomes a different design rather than a deeper trench.

Every depth also has to clear whatever is buried on the line. That is a locate call before a shovel, not after.

More on this: Exterior French Drain and Waterproofing in Syracuse, UT.

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