Under some circumstances, it isn't possible to plan deeded structures on cobblestone or similar "road" tiles– Instead, you're met with this line: "You are not allowed to build structures on roads outside of settlements." Whether or not you can build seems to depend on where you are in relation to the tile being planned on. Standing on the tile in question seems to always work, but adjacent tiles are either inconsistent or don't work at all. Tested under these conditions: Attempting to plan on deeded cobblestone tiles (other tiles such as floorboards and grass were unaffected) Playing as the deed mayor (no building limitations) Not near deed borders Not touching other walls or structures Not near any "monolithic" structures such as colossi or guard towers All ground and its surroundings were flat Plenty of carpentry skill Zero other work-in-progress structures