CASTLE ROCK — When the first houses go up in the Dawson Trails neighborhood in Castle Rock in a couple of years, it will mark the beginning of the end of metro Denver’s decades-long southward migration.

This fast-growing town in the heart of Douglas County has no plans to annex any land farther south for development, meaning that what lies between Monument and Castle Rock today — vast expanses of open space and natural landscape along with a few dispersed ranchettes — should remain that way for future generations.

In other words, there is a place where one can escape the clutches of Denver and the suburbs that encircle it. A place where the prospect of more homes and more shopping centers and more bumper-to-bumper traffic finally dies.

There is, in fact, an end to metro Denver.

“That’s as far south as the Douglas County master plan shows any urban development happening, outside the town of Larkspur,” said Steve Koster, assistant director of planning services for Douglas County.

With fewer than 300 residents, Larkspur has no expansion plans in the pipeline. And if it ever does, it will be largely hemmed in by large swaths of land protected by conservation easements and preservation designations — the result of the purchasing power of Douglas County’s nearly 30-year-old open space sales and use tax.

The county currently has more than 64,000 acres off-limits to development via open space and easements. Voters will decide this November whether to extend the tax for another 15 years.

“For at least the foreseeable future, that does limit the growth to the south,” said Phyllis Resnick, executive director and lead economist with the Colorado Futures Center at Colorado State University.

But the natural buffer between Castle Rock and Monument — and in a larger sense between Denver and Colorado Springs — in no way means that urban sprawl is a done deal in metro Denver. The state demographer’s office forecasts that Douglas County alone will have nearly 400,000 residents by 2025 and more than 450,000 by 2040.

Growth is exploding in Lone Tree, Parker and Castle Pines, which broke ground on a housing development at the end of 2019 that will double the size of the town directly north of Castle Rock.

The metro area as a whole, which has just over 3.2 million people now, will grow to just under 4 million in the next 18 years, according to the state demographer’s office. That compares to a population of 1.8 million in 1990 and 2.4 million in 2000.

Those new arrivals will have to go somewhere, even if they don’t head deeper into Douglas County. Resnick sees a sharp contrast between the southern edge of the metro area and its north or east sides, where she said further sprawl could happen largely unimpeded unless large tracts of land are placed under some form of protection.

Aurora has numerous large residential and commercial projects in the works, some of which stretch east of the E-470 beltway. To the north, development in Adams, Larimer and Weld counties along I-25 shows no signs of stopping.

“It’s likely happening that the metro area will one day sprawl all the way to Fort Collins,” Resnick said. “It’s kind of like a balloon — if you squeeze it in one place, it’s going to pop out over here.”

The foothills act as a natural barrier to runaway development on the west side of town, she said.

But even if Douglas County has created a hard stop of sorts for development south of Castle Rock’s town line, it doesn’t mean the town itself has jettisoned the earth movers and drywall hangers inside its boundaries: Castle Rock’s population of 81,000 is expected to top out at up to 140,000 residents over the next few decades.

“The key limiting factor is water,” said Town Manager Dave Corliss.

Dawson Trails, itself, comes with 5,850 homes and a 158,000-square-foot Costco, with its popular fueling station, along with more than 3 million square feet of additional commercial and office space. Costco is expected to be Castle Rock’s largest single tax revenue generator, at $4 million to $6 million annually, Corliss said.

The project will be topped off with a brand new $118 million interchange on I-25 — the Crystal Valley interchange — which is expected to break ground next year and wrap up in 2025.

“It’s currently got some cows on it,” Castle Rock Development Services Director Tara Vargish said of the 2,000 acres on the west side of I-25 where Dawson Trails will rise.

Corliss said Castle Rock has planned for Dawson Trails — formerly known as Dawson Ridge and before that as Castle Rock Ranch — for nearly 40 years, when the land was first annexed into the town. For a number of reasons, including a developer bankruptcy, nothing happened on the parcel.

But this month, the town council gave final approval to the project. Completion of Dawson Trails could take until 2040.

“This is the end of Castle Rock’s plans for southern growth,” Corliss said.