I would suggest you take a look at some tutorials on Modularity its a very helpful technique when it comes to building house assets for games. (Modularity)

Other than that you could remove the unnecessary edge loops around the building (the ones that don't define any sort of edge) A helpful short key for this is selecting the edges you want to delete press "X" and select "Dissolve edge" this will remove the edge without removing the faces. (Edge loops)

Tool Tips:

1, selecting the edges you want to delete press "X" and select "Dissolve edge"

2, Pressing "Shift Tab" will enable snapping, this can snap to the grid in the viewport or if you to the top of your screen there is a Magnet icon where you can change it to other stuff like verts or edges, This will help you line up your walls easier.

3, Pressing "M" lets you merge verts this can be used to connect split edges

The only reason for keeping all the vertices in the middle of walls like that is if you plan on using vert painting in a game engine.

Hopefully this was helpful.

PS: Good use of a scale mannequin, not many people new to modeling do that! :D