New storm water rules taking shape in Ventura County have public and private builders concerned that costs for routine projects could go way up and that some developments — such as large factories or warehouses — wouldn’t be feasible at all.

The big change in the rules is that builders will have to deal with almost all of the rainfall that hits their property at the site, either letting it soak into the ground or capturing and treating it rather than directing it to the county’s storm water drainage network. The new regulations were approved in May, and officials are figuring out how to implement them.

Much about the new rules remains unclear even to close observers, but a few certainties have emerged: They’re likely to be costly, and no one is exempt. Across Ventura County, roads, schools and hospitals will bear any added expenses along with housing subdivisions and factories.

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