Pre-Development Impervious Area: A Quirk in the NYC Stormwater Manual

Whether a project needs a stormwater construction permit and SWPPP often starts with two numbers: how much soil it disturbs and how much new impervious area it creates. Disturbing 20,000 sf of soil or creating 5,000 sf of new impervious area — either one alone — triggers the requirement. Under NYC's Unified Stormwater Rule, that's the definition of a "covered development project,"[1] and it applies citywide, to public and private work alike — not only within the street right-of-way.

The impervious-area figure, though, depends entirely on how you define the pre-development condition — and on that point, the NYC Stormwater Manual (SWM) isn't consistent with itself.

Two definitions, in tension

The SWM's glossary defines impervious area to include:

"…any portion of the site of a covered development project from which impervious cover was removed within five (5) years before SWPPP submission to the Department."[2]

In other words, if a building stood on the site within the last five years and was demolished before you applied, that footprint still counts as impervious for baseline purposes. You don't get to treat a recently-cleared lot as if it were always vacant.

But when the manual actually walks through how to calculate the pre-to-post difference, it says this instead:

"The pre-development case must represent the least amount of impervious surface for the disturbed area within the last 5 years prior to proposed development."[3]

Read literally, that's the opposite instruction. "Least amount within the last 5 years" points you toward the lowest point in the window — often the moment right after a demolition, when the site is at its most cleared. That directly contradicts a glossary definition built to prevent exactly that outcome.

Which one actually governs

In practice, the standard that gets applied is the glossary's version: pre-development impervious area reflects the highest documented cover on the disturbed area within the five-year lookback, not the lowest. DEP's own SWPPP Template backs this up: it calls for a "Historical Impervious Area Plan" whenever "historical cover within the last 5-years does not match current surveyed conditions,"[4] delineating any impervious areas that were removed from the site within that window. That requirement only makes sense if historical cover is meant to be credited — if the "least amount" reading actually governed, current surveyed conditions would always be sufficient on their own. A structure demolished two years ago still counts, even if the lot is vacant at the time of SWPPP submission.

That's also the version consistent with the manual's evident intent. The five-year lookback exists so that a site isn't penalized for a demolition that happened before the survey — not so an applicant can manufacture a lower baseline by clearing a site first and applying second.

Why it matters before you rule a project in or out

If you calculate your new impervious area using only the current, as-surveyed condition of a recently-cleared site, you'll typically overstate how much impervious area is actually "new." That can push a project over a threshold it wouldn't actually cross once the historical condition is properly credited, or inflate the sizing for stormwater management practices beyond what's really required.

The catch is documentation. Crediting a historical high point requires evidence — aerials, old site plans, permits, or dated photos showing what was there within the five-year window. Without it, expect a reviewer to default to current conditions, which puts you back where the literal "least amount" reading would leave you.

Worth noting: the Stormwater Manual is a living document, and specific language can change. Always confirm current guidance for your project rather than relying on any single read of the text — including this one.