"build" is a verb
adding "ing" to it makes it the result of the verb itself, iow: the finished product, i.e; "building"
and consequently turns the word into a noun.
similarly, if you've for example, tiled your bathroom, and someone wishes to say how well you've done that job(i.e; the end result of having to "tile" that room) , they would compliment your "tiling" as opposed to complimenting your "tiled"
pouman