creep.moveTo(constructionSite.pos.x, constructionSite.pos.y);
console.log(creep.build(constructionSite));
returns -7
creep.moveTo(constructionSite.pos.x+1, constructionSite.pos.y+1);
console.log(creep.build(constructionSite));
returns 0. As a builder, it should be possible to start constructing on a site too.