Variant of the phrase "broad brush strokes"

rough-hewn

rough-hew
tr.v. rough-hewed, or rough-hewn
1. To hew or shape (timber, for example) roughly, without finishing.
2. To make in rough form.
American Heritage Dictionary

I like this term because it seems to match your case. It implies getting timber that's very roughly cut, and that possibly will be refined later. So using your sentence:

At this point, it's sufficient to make a rough-hewn drawing/plan/draft. We can refine it later (my addition).

Usage note: rough-hewn may or may not be understood by people whose English skills are a bit... wanting, so if you don't want to take any risk whatsoever in miscomprehension, you may want to use another term.