In-game world building?

Is there a way to add in-game world building and editing? I'm interested in making a base grid-based base building game- think of Sims with its grid and auto-connecting corners in build and buy mode (ie the walls and counters respectively). It seems like the concept of using a grid wouldn't be hard once I set it up, except for that my walls would have to use a different grid than the ceilings, walls, and floors, and that the grids would then become huge as the world expands to include new features. Additionally, editing the ground will most likely be an issue, as I don't think terrains are move after the game starts (removing any potential ability to create a basement or other structure). The only alternative I can think of is using various-sized angles and cubes to edit the terrain with, and have the other objacts align themselves based on their grids and the cubes' heights- like in Roller Coaster Tycoon.