The conventional order of operations places buildings, then parking, then finds somewhere for the stormwater management facilities. On a flat, dry, unconstrained site this works. Those sites are largely gone.
Infiltration requires soils that infiltrate and a seasonal high water table far enough down. Those conditions occur in specific places on a parcel, and they rarely coincide with the leftover corner. When the basin gets pushed to leftover ground, the usual outcomes are an oversized surface basin eating developable area, or an underground system costing several hundred thousand dollars more than a surface one would have.
We run test pits and infiltration testing during due diligence, before the first massing study. The results become a layer on the constraints map, and the site plan is drawn around them. On a recent 38-acre logistics project this approach recovered enough developable area to justify the entire engineering fee several times over.