Stormwater is a non-issue and generally a waste of money racket unless you're talking about developing a many acre concrete jungle or some sort of facility that will generate pollution that it may pick up. Datacenters, warehouse, etc. don't really have that issue. "Treatment" in these cases would consist of the gutters simply running into a bioswale (i.e. a grassy ditch, god I hate how the industry intentionally makes the language incomprehensible to laymen). There'd be some number crunching regarding volume and stuff but nothing that any competent dirt work contractor couldn't shoot from the hip simply by looking at the building size. There's no real cost impact to over-sizing it once you've set down the path of engineered site plan which all these facilities probably will.

The wastewater I am concerned about though for the thermal reasons many have cited.