Making my building blocks modular I'm modelling the different parts of my scenes as modular building blocks. This way, I can make up different rooms/scenes starting from these different blocks. Basic building blocks are:

Wall:



Floor tile:



Door:



Open corner:



Closed corner:



There are more building blocks, but these are the basic ones. Main problem is that, when I build a scene, I somehow end up being unable to close my rooms using ONLY these building blocks. In the next screenshots you will appreciate this phenomena:




As you can see, it is impossible to close the red gap with a basic wall building block, without having an overlap between them.

Much in the same way, you can appreciate that the basic floor tiles building block will get past out of walls.

I have to choose between mantaining reusability (via building extra helper building blocks), or breaking this reusability by directly modifying each room separately.

Can you come up with a solution? I would really like to keep re-usability, since I'll be building dozens or more rooms, and I wouldn't really like to modify each of those independently.

Here are more screenshots of the room I'm building right now:




EDIT:
Solved! Thanks to @Carlo's indications about the Open Corner block, now my two problems have been solved very easily. In a nutshell, the problem was that I wasn't having into account the thickness of the Open Corner building block. Here is a screenshot of the new scene:
Modular assets need quite a lot of planing. First you need to specify a grid size. This might be a floor tile, it also might be bound to a certain measurement like the width of a door frame. Measured from the borders of your base grid, you then need to inset the interior walls and outset the exterior walls. The gap between will be the thickness of the wall. 

The next point you need to consider carefully, is the center of your wall. If your grid starts at either the inside or the outside, your modules won't have the right size. You want to have Blender's grid lines exactly in the middle between your inner and your outer wall (see the green lines in the screenshot below. Also notice the slightly thicker lines of Blender's major Unit grid). 



My walls are exactly two Blender Units wide. The floor tiles will then be perfect squares and also have two Blender units in width and height. The floor will overlap with the wall but it won't show from the outside, since it goes only to half the size of the wall's width. 



There you go, perfectly matching tiles in all directions. 

Here's a .blend file with a very basic setup.



PS: You also want to remove all hidden faces. They are unnecessary in nearly every setup, as long as you don't want to create gaps in the wall, where the sides are visible. The floor tiles in my file are just planes but the principle is the same as it is for the walls.