A project for building in game.
Left Mouse Clicking anywhere on the map will set your build position. Clicking on a block face will set the build location of the next block to start on that face/plane. The F key places a new block. Y and I move the selected block in Y positive or negative. U and J move the selected block up or down in Z. H and K move the selected block in X positive or negative.
Snapping is rudementary, a greater system needs some design decisions regarding grid construction and size, sockets consideration, piece shapes, etc.