NEW HAVEN, Conn. — The bell tower looming over a bustling seven-acre construction site here on Prospect Street signals a major development for Yale University: the imminent debut of its first new residential colleges in a half-century.

When Franklin and Murray colleges open in August, they will raise the capacity of incoming classes 15 percent, to about 1,550 seats a year. That will enable Yale’s undergraduate enrollment to grow from about 5,400 now to 6,200 over the next four years.

For Yale, it will be the most significant expansion since women were admitted to the undergraduate college for the first time in 1969, a milestone that led to a 22 percent enrollment increase in that era.

Franklin and Murray, with an elaborate collegiate Gothic design echoing other parts of the campus, will become the 13th and 14th residential colleges in the network of living and learning communities within Yale College that began in 1933. They will be the first to open here since 1962.

Adding 200 seats a year would amount to a rounding error for fast-growing public universities in places like Florida, Texas and Arizona, where enrollment is measured in tens of thousands.

It will probably make little change to Yale’s admission rate, which is now 6 percent, one of the lowest in the country. But the symbolism of an ultra-selective Ivy League school, founded in 1701, growing 15 percent at the outset of its fourth century is significant. The nation’s top private colleges and universities are often slow to expand despite huge global demand.

Why has it taken 50-plus years for Yale to build more residential colleges when applications for admission now exceed 30,000 a year?

“One can only say that Yale changes slowly,” Jonathan Holloway, dean of Yale College, said in a recent interview.

One can also say that it takes a lot of money. So much that the expansion, approved in 2008, was delayed several years because of the global financial crisis.

Plans accelerated anew in 2013 after Yale announced a $250 million gift from alumnus Charles B. Johnson to fund the project. In 2014, Yale President Peter Salovey declared that the university had raised enough money in a $500 million fund drive to start construction.

“When Yale builds new colleges, it’s not an inexpensive proposition,” Holloway said. “It’s radically expensive. . . . Our residential spaces are rather over-the-top. You name it, each college has it.”

That means dining halls, libraries, gyms, living quarters for the head of each college and the dean of each college, off-hours kitchen and snack areas called “butteries,” common rooms, courtyards and more. Some colleges have performance spaces for dance and theater. Some have squash courts.

Building Franklin and Murray required obtaining vast quantities of granite, brick, limestone and slate for the exterior, oak for interior hardwood floors and various other elements — arches, gates, bay windows and the like — to create a polished look that aims to blend into the rest of the historic New England campus.

The colleges were named for Benjamin Franklin, one of the nation’s Founding Fathers, and Anna Pauline Murray, a Yale alumna who was a scholar, lawyer and civil rights leader and the first African American woman to be ordained as an Episcopal priest.

Johnson, the major donor behind the project, was said to have suggested Franklin’s name. Still, the choice was somewhat controversial here in part because Franklin owned slaves as a young man before voicing opposition to slavery later in life. Yale also has been wrestling lately with whether to rename a residential college that now honors the 19th-century politician and slavery advocate John C. Calhoun. A decision on that question is expected early next year.

Building the two residential colleges is hardly the only step in expansion. Yale has added professors in recent years, growing from roughly 600 tenured and tenure-track positions in the faculty of arts and sciences toward a goal of 700 by 2021. There are now about 650, said Tamar Szabo Gendler, dean of that faculty unit.

Yale also has been planning how to expand student services and course offerings and schedules to ensure that it maintains small classes for undergraduates.

Holloway said consultants have helped Yale study its classroom use and “room flow.” The university has enough space, he said, but orchestrating which classes will be held when and where is a delicate political matter. At many universities, professors have little say in those issues.

Not so at Yale.

“Faculty are not accustomed to being told, ‘You’re going to teach this class on this day at this time,’ ” Holloway said. “That just doesn’t happen.”

Expanding faculty is, of course, expensive. Which leads to another point: Many colleges and universities these days grow their student body explicitly to net more tuition revenue. Yale officials insist that was not their goal.

“At no point in the expansion did we think we’d do it to raise money for the university,” Yale Provost Ben Polak said. “This is about access.”

Providing a world-class education to more students is a good thing, Polak said. But there are limits. “We can’t expand infinitely,” he said.