Hi Mimi, as David noted, this detail does substantially increase labor costs, so your builder would be justified in charging more for this. We have done this twice, (on ceilings and floors) and both clients have regretted it, because the walls show far more wear from vacuuming, mopping and anything that bumps into it. (Plaster would be more forgiving of course)
Because of this, you may want to consider a slightly different detail which is more practical and less work:
This detail does include baseboard but it is flush with the drywall face so it offers a cleaner look. It involves less labor than a small drywall-only reveal does, as it does not require the floor to be perfectly flat for it to look good.
Another option is to use whats called a "reveal base" drywall bead, which is taller than the "f reveal" which is most commonly used at the floor. With the reveal base your shadow line is taller and the exposed part is metal so it has more durability. Its painted the same color as the drywall. This detail is a fairly common in more contemporary commercial projects.
For more drywall bead profile options, try Fryreglet.com, they are one the biggest suppliers for these products. Another great one for modern projects is tektrim.com
Hope this helps.
Steve
Q