School officials in Loudoun County, the only district in the Washington region without universal full-day kindergarten, say that it would be too costly to add extra classrooms and staff in the fast-growing county. But nearby, Prince William County doubled the school day for all its kindergarten students in a few years.

The rollout happened just before the housing market collapsed, and the county used portable classrooms in many schools to make it more affordable, said Prince William Superintendent Steven L. Walts.

In a recent interview, he said that making full-day kindergarten universal was his “greatest accomplishment.”

Here are some highlights from our conversation about full-day kindergarten and early education.

Why do you consider it your greatest accomplishment?

I feel like I did every subsequent grade level a favor. . . . The earlier we have those opportunities, the more value you can have. There is huge progress you can make at this age because of the developmental connections the brain makes.

If you ask the parents, the satisfaction of full-day kindergarten is off the map. Parents love it.

Why make it universal and not just for children who are at-risk and would likely benefit the most?

We don’t look at it that way at any other grade level. The fact is if we are interested in the growth and development of all children, you take kids who are doing well and take them farther.

How much compelling research does their need to be to show that the value far outweighs the cost of early education.

Why not invest more in preschool then? Prince William County provides state-funded pre-kindergarten classes to only 72 students, even though the state would provide matching funds for more than 1,600.

It’s hard to expand something when the state is giving you cuts and the local board is giving you cuts.

I think it’s highly valuable, but I do support what the majority of our board says with regard to the cards we’ve been played financially. There’s not a pot of money sitting around that’s big enough to address this pre-K [issue], save this pot of state money that’s sitting around with all kinds of strings attached [eg., the required local match].

If that money had the restrictions lifted, we could use it to build pre-K classrooms.