Structures are special objects that serve a passive purpose such as using a Wall to defend or blockade an area, or an active purpose, such as a Houndius Shootius being used to protect an area against mobs. They are either crafted by the player or, more rarely, found around the world.
When placed, every structure or plantable has a radius. Nothing new can be placed inside this radius unless the structure is destroyed. It doesn't take into account what is already there; it only counts the radius of the object being placed. For example, a Chest can be placed very close to an existing Birdcage, but if a Birdcage is built near an existing Chest, it will need to be placed significantly farther away.
The following are the radius values from the LUA files:
|Houndius Shootius||0.00||Can be placed anywhere on valid ground.|
|Walls||0.10||Distance from other walls.|
|Walls||1.00||Distance from things that aren't walls.|
|All Other Plantables||2.00||Things that can be items.|
|All Other Structures||3.20||Things that can't be items.|
Note on Plantables vs Structures: For the purposes of the table above, if you can build it and it goes into your inventory, it's plantable. If you can pre-build it and place it later, it's a structure.
If looking to build many things in a small area, prioritize structures with a large radius (For example, build Crockpots first, then Ice Boxes and finally Walls).