What’s the Urban Development Boundary, and what does it mean for Miami-Dade County?

For decades, some of the biggest fights over construction, sprawl and the environment in Miami-Dade County have involved something invisible that literally only exists on paper: the Urban Development Boundary.

Here are some questions and answers to help understand the role of the “UDB,” why it’s a source of perpetual conflict between developers and environmentalists and what it means for Miami-Dade County housing.

What is the Urban Development Boundary?

The Urban Development Boundary is a legal divide on Miami-Dade’s land-planning maps that governs how much construction can occur on a piece of land. It’s about 78 miles long.

Outside the boundary, county rules limit residential construction, typically to single homes surrounded by five acres of land. Inside the boundary, the rules are far looser, ranging from suburban subdivisions with six homes on an acre to high-rises in downtown Miami with hundreds of apartments.

What about commercial construction?

The UDB serves the same purpose for commercial construction, with strict limits outside of the boundary and a range of looser rules inside it.

The primary purpose is to encourage development closer to existing houses and businesses, where roads, transit lines, schools and other government services already exist. The UDB also serves as a buffer between development and environmentally sensitive lands around the Everglades. Most farms operate outside the UDB, and the boundary is designed to shield agricultural land from development.

Can it move?

Yes. County rules governing the UDB say it should move once Miami-Dade has fewer than 10 years’ worth of land to meet construction demands within the Urban Development Boundary. The county uses forecasts on population growth, construction and demand for residential and commercial real estate to predict when land with those categories would be depleted based on existing trends. The forecasts are adjusted multiple times a year.

So once the county is out of vacant lots, the UDB moves?

No. The county also considers whether land within the UDB could be redeveloped into a project that utilizes more of the acreage than exists today — such as a lot with a townhouse community that’s zoned for a high-rise tower. There are already areas outside the UDB designated as “Urban Expansion Areas,” where the boundary should move once land runs short inside the UDB.

Can the UDB move even if land supplies aren’t exhausted?

Yes. The county commission can opt to move the UDB based on other considerations, and can look at land supplies near a proposed project.

How long has Miami-Dade had the UDB?

Since 1983.

Has it moved since then?

Yes. The last time was in 2013, when county commissioners added about 500 acres for an industrial project in the Doral area.

What’s on the other side of the UDB?

Lots of farmland. Also rock-mining operations, wetlands, the federal Everglades National Park, and some well-traveled roads, including Krome Avenue.

Who can move the UDB?

County commissioners, with a two-thirds vote on the 13-member board.

Didn’t that just happen?

No. On Sept. 9, commissioners in a 9-3 vote agreed to advance an application to expand the UDB near Homestead to construct a 9 million-square-foot project called the South Dade Logistics and Technology District industrial park. That vote forwarded the application to Florida regulators for review. A final vote is required by the commission to extend the UDB, and that could come later this year or in 2022.