When Two Lots Become One Problem: The Stormwater Question Developers Should Ask Before They Buy

When a developer buys a single-family lot to build a multi-family project, the numbers usually pencil out before the civil engineer weighs in. That's a problem — and one Keith Matte has seen more than once.

On a recent project in St. Paul with developer D/O, the team purchased two single-family lots and planned to combine them into a single multi-family building. What looked like a straightforward land-use conversion quickly revealed a significant infrastructure gap: there was no public storm sewer main within 300 feet of the site.

That's not just an inconvenience. Under Minnesota's plumbing code, a multi-family building must connect directly to a public storm sewer main. When that main doesn't exist nearby, the cost to comply can derail a project entirely.

The Options the City Gave Us

BKBM Engineers' first instinct was a sidewalk chase — a simple tunnel beneath the public sidewalk that allows stormwater to flow from the site into the street. It's a solution the firm has used in other municipalities. Cost: roughly $30,000. The City of St. Paul said no.

The city's alternative? Capture the back-to-back 100-year storm event on site — meaning the site would need to retain the equivalent of two consecutive 7.8-inch rain events in 24 hours. The infrastructure required to do that on this site, given its clay soils, meant a 9-foot infiltration pit filled with rock, connected via a 14-foot sand channel to reach a usable soil layer beneath. Estimated cost: approximately $500,000.

For comparison, digging up the street to install a new public sewer main to the nearest connection point would have cost $500,000 to $750,000.

Neither option was workable for a small infill development.

The Conversation That Changed the Project

Rather than presenting the owner with a $500,000 problem and calling it done, Keith went back to the city. His relationship with St. Paul's surface water team opened a door to a direct conversation with the city's lead plumbing inspector — a conversation that changed everything.

The inspector agreed to allow stormwater to drain into the adjacent alley, flowing naturally toward the public main. It wasn't a loophole. The code allowed it. St. Paul typically enforces a stricter internal interpretation, but because the site was a small infill development with no nearby public infrastructure, the inspector was willing to apply the code as written.

There was still a design challenge: the alley sat at a higher elevation than the building's floor. Water doesn't flow uphill. Working with the architect and BKBM's structural engineers, the team raised the building and constructed retaining walls along the rear so the finished grade would allow drainage toward the alley. The city also permitted roof drains to discharge above finished grade — another exception to standard practice, granted because of the infrastructure constraints unique to this site.

The final solution cost approximately $100,000 — a fraction of what the city's original options would have required, saving the owner roughly $400,000 or more.

What This Means for Developers

The D/O project is not an outlier. In urban infill markets like St. Paul and Minneapolis, single-family neighborhoods are a primary target for multi-family redevelopment — and those neighborhoods were not built with storm sewer infrastructure in front of every lot. Catch basins and storm sewer mains tend to be located at intersections, not mid-block. If your site doesn't sit near an intersection, there may be no public storm sewer within reach.

The question every developer should ask a civil engineer on day one — before purchasing the parcel — is simple: Is there accessible public storm sewer infrastructure at this site?

That one question, asked early, can shape the entire pro forma. Engage a civil engineer for a site feasibility review before you close. A few thousand dollars of due diligence can keep a half-million-dollar surprise off your balance sheet.

BKBM Engineers provides civil and structural engineering services across the Twin Cities metro and beyond. Keith Matte specializes in civil site engineering for multi-family, mixed-use, and institutional projects.