Dan Evans, former governor of Washington and a former U.S. senator, and his wife, Nancy Evans, are in the middle of building a house—a small but ambitious modern design with lots of angles, a metal roof, and walls of glass.

Mr. Evans is 95 years old. Mrs. Evans is 87.

“It’s fun but a lot of work,” says Mrs. Evans, who declined to disclose the cost of the construction. Mud from heavy rains can make the site difficult to access and Covid restrictions complicate shopping for fixtures and materials.

The couple have owned the property, north of Seattle, for more than 40 years, but say they have always been too busy or didn’t have the money to build. Now, they want to create a space for their three sons and their grandchildren to gather. They hired the architects in 2018. The walls are up and they are working on the kitchen cabinets now. They are hoping it will be done in early June. “You have to do something at our very ancient ages,” jokes Mrs. Evans, who, along with her husband, is on numerous boards and foundations.

Architects across the country have noticed older clients are increasingly taking on new construction and major renovation projects. It is partly because there are more households led by people 65 or older, with a million added nationwide every year between 2014 to 2019, according to the Harvard Joint Center for Housing Studies.