In the new house, our his & her master closets combined are much smaller than the one ginormous closet we shared in the old house. This has become a problem. Now, my own closet requirements are pretty low; a single rod and a space on the floor for a few pairs of shoes will do. What I really need are a few huge drawers for my infinite T-shirt collection. But Mrs. has shirts that need hanging, shoes in shoe boxes, pants that she doesn't want wrinkled, dresses of all length, etc.. Sure, she can use all the room in mine that I'm not using, but that's not optimal. And, it really doesn't fix the problem. For, not only is her new closet small-ish, the layout of the shelves and hanging rods in the new place was a) not chosen by us and b) really rather poorly thought out. Rods and shelves overlap, vertical spacing is too short, not enough shelves, etc.. Lots of wasted space. This has been a sore spot for her.
So, after a couple of months of struggling to make it work, Mrs went out and hired a closet designer to come in and redo her closet. Took them just a few hours to rip out all of the old stuff and install a new system of movable shelves and rods that gives Mrs more (and hopefully enough) places to put all her stuff. Now, it's definitely better laid-out, and it's flexible if she wants to change.
So... if you're building a house, and someone in your family is more demanding of closet space than I am, it's probably worth it to spend some time with someone who knows what they're doing, when it comes to designing the closet space.

I used to clean a lady’s house in Seattle that had almost no closet space. She was pretty well off and just bought a bunch of fancy wardrobes. Which were pretty cool but ate up a lot of the square footage of the house.
I got a real kick out of closet design. We used some layouts from EasyClosets.com as a jumping off point, then had our carpenter do the actual buidling. I had never seen or thought about some of the techniques they use to maximize space — my favorite is “double hanging” where you have a rod halfway up the wall and another 40 inches above it.
Our shared closet is 7′ x 5′, which is not bad, but we’re still overflowing badly. I have too much stuff.