Level 2 Garrison building plans are not useable after completing appropriate outpost quests in Gorgrond, Talador, etc (Vendor in Warspear / Ashran where you turn in the Construction guide / Assembly notes quest rewards)
I see that the developers took a shortcut in ‘fixing’ the issue. (Originally the plans said useable at lv 10, but they really weren’t useable until level 40) Plans now say that they require level 40, (which they do) but that’s not the way it is supposed to work. We can build a level 2 garrison at lv 20 whether we have done the appropriate quests or not, (Requirement is level 20 OR ‘foothold’ quests) so level 2 plans should be useable at level 20 at a maximum.
Note: The plans you buy from vendor in your garrison have different requirements (Different level/zone requirement) and this is ok since it would most likely be the 2nd set of large / medium buildings you acquire and wouldn’t be able to use those until level 3 garrison anyway)
At level 40, we can build the level 3 garrison, so it doesn’t really make sense for the level 2 plans to finally become available during level 3 construction.