Unity: Make a tile take up more grid cells - unity3d

Question

I am trying to create a basic tower defense game where I have set up my cell size to be 0.5 X and 0.5 Y (so that the player can place their towers more freely, think WC3).
This causes problems when I later in the game wants to check if a grid cell is occupied, because some cells will seem to be taken, but actually are not.
Here's an image to illustrate my problem:
The black square is rendered over 4 cells, but only 1 of the cells are occupied (the white square in the lower left corner of the black square).
Have anyone else faced this particular problem and knows how to solve this or are there any other solutions that you would like to recommend? :)
Thanks in advance!

Answer 1

Though I'm unsure how the unity Grid component works I have used the following approach in one of my grid based games. Note however that I implemented my own grid for this, which contain custom grid tiles. But maybe the same logic can be applied to the unity grid
This grid was just a simple grid made like this
for (int i = 0; i < terrainLength; i++)
{
for (int j = 0; j < terrainWidth; j++)
{
GameObject tile = Instantiate(gridTilePrefab, new Vector3(posX + i * gridCube.transform.localScale.x, posY + terrainHeight, posZ + j * gridCube.transform.localScale.z), Quaternion.identity);
tile.name = "grid[" + i + "," + j + "]";
tile.transform.parent = gridParent.transform;
}
}
These grid tiles would have a boolean isOccupied. That would be set to true if an object is placed on it, and false if not.
To check wether or not it was occupied I would simply cast a raycast up from the center of the tile and check for any collision while in the builder phase (No need to do these checks during play!) the implemented was a simple as this:
Class GridTile
{
public bool isOccupied {get; private set;}
public void BuildStageLoop()//this loops like an update while we're in building stage
{
if (Physics.Raycast(transform.position, Vector3.up * 2, out hit))
{
tileOccupied = true;
}
else
{
tileOccupied = false;
}
}
}
And on the placing object I would just check if every tile underneath it had isOccupied set to false. To check for the tiles underneath it I would do a boxRayCast downwards with the width and length of the object you're trying to place, and extending a bit underneath the object so it can collide with the grid tiles.