[LE] Can't Build On Highway

[WhoppingTurd]

I've created a custom settlement/base area on an elevated highway next to Outpost Zimonja. Building and the workshop works, however I can't build anything on the top level, like I'd be able to at the Graygarden settlement of the same type. Is there some way I can add a "ground level" as such so I can build on the top road?

I've also cleared some rubble and debris piles within the creation kit to give the area a cleaner look. Would this impact the building function?

 

The red outline stays no matter how far up/down/left/right

[WhoppingTurd]

Edit: Just realised I posted this in Skyrim rather than Fallout. Derp.