This mod lets you place buildings anywhere in your land, including within other buildings.

Installation
Download the latest version from the releases page
Place the downloaded zip in your Documents / My Games / FarmingSimulator19 / mods folder
Launch the game and enable the mod in the mission start screen

You can change the cost of terrain modifications by opening PlaceAnywhere.lua in the mod’s zip file and changing the value of
PlacementScreenController.DISPLACEMENT_COST_PER_M3 = 1; — Edit this to change the terrain modification cost per cubic meter (game default: 50)

I’ve tested this mod and didn’t notice any side effects, however it might still cause issues as it removes some game checks.

Changelog
1.1: Fixed bug when buying vehicles
1.0: Initial release