In Houston, Texas, the design firm Hannah is working to build the first two-story 3D-printed home in the U.S., one layer of concrete at a time. The home is one of several 3D printing construction projects completed or currently underway in a handful of states.

3D printing in construction has existed for more than a decade, and the process is making big strides outside of the U.S. France has plans underway to create five 3D-printed social housing residences, and another project in Gananoque, Ontario, is expected to consist of up to six multi-family homes. Meanwhile, in California, the homes that make "Mighty Quatro" 3D-printed village are actually fabricated in Monterrey, Mexico before being sent to the site for construction.

According to the co-founder of Hannah, Leslie Lok, the U.S. homebuilding industry has only started to adopt 3D-printing in the last few. For her and many other supporters of 3D printing, the ultimate goal is to find a low-cost way to build durable multi-family homes.

"This Houston project is a step towards that, being a pretty large single-family house," Lok said.

The Houston project might be the tallest so far in the country, but it's far from the largest. Iowa City-based Alquist 3D is underway on its plans to build 200 3D printed homes around Iowa, with additional plans to build homes in rural areas in Virginia, Florida, Missouri, Kansas, and Texas.

To construct Hannah's 4,000 square-foot, three-bedroom home, a giant machine has been pouring concrete out of a nozzle virtually non-stop since last July. The home is being built by Hannah in collaboration with another Houston-based company, Cive engineering and construction, and Peri 3D Construction based in Germany.

The team hopes to use the Houston project to find new ways to streamline the 3D printing construction process. The aim is to develop a method for quickly building well-designed homes.

"In the future, it has to be fast, simple design in order to compete with other building technologies," Cive's head of structural engineering, Hikmat Zerbe, told NPR.

For now, speed isn't the goal. Instead, the builders are using the project as one "big laboratory" for 3D printing a home.

"We are not trying to beat the clock," Zerbe said. "It's a case study. We're learning the capabilities of the machine, learning the reaction of the material under different weather conditions. We're learning how to optimize the speed of printing."

According to Zerbe, the project in Houston is nearly halfway done. Currently, the preparation and installation costs for 3D printing homes are high, much higher than traditional homes. However, Zerbe says improvements in the process could eventually make 3D printing printing more cost-effective than traditional construction.

Lok, who was the architectural designer for the Houston project, sees cost-saving opportunities on the design side of things. She says 3D printing could allow builders to offer personalized options for buyers on a large scale while keeping labor costs low.

"The printer doesn't care if you print the same chair 100 times or you print 100 different chairs," Lok told NPR. "This opens up the possibility of how we can actually offer customized design for the users, whether it's a single-family house or whether it's a multifamily building or apartment."