Residential high-rise buildings in downtown Phoenix and basement homes

According to the U.S. Census Bureau, between 2010 and 2020, the population of Phoenix increased by more than 18%. It is now ranked fifth by population in the United States, edging out Philadelphia by about 130,000 people. Its land area, however, exceeds that of Philadelphia by about 383 square miles.

Five years ago, the population density of Phoenix was similar to Austin and Atlanta, according to the census data. Phoenix’s population density was almost four times less than Philadelphia, and roughly 10 times less than New York, which ranks first in population.

By population and land area, Phoenix may not be considered dense, but its ground is.

Unlike cities in the Northeast, the Valley’s desert soil is often riddled with caliche, “a crust of calcium carbonate that forms on the stony soil of arid regions.”

Caliche is a hinderance to home builders and can limit home designs, like the addition of a basement. Ramon Arrowsmith, a professor of geology in the school of earth and space exploration at Arizona State University, say there is plenty of space, and instead of digging deep, just build horizontally. Others like Will McCrory, a local architect, say more cities are looking upward.