Building Size and Skill Level
The formula to calculate the carpentry skill needed to build a single story building is Number of tiles + the number of outer walls.
Each floor above the first floor requires a certain skill level.
Stone buildings require at least 30 masonry. You'll need to make bricks anyway, which will skill you up to that point.
NOTE: WO's formula for a single floor building is (Number of tiles + the number of outer walls)-5