It's now time to refine some elements in order to give them an old/destroyed look. This will mainly come from the texturing work that we will do later, but we can still work on the geometry a little bit:
We will begin by working on the stairs. We will delete the side faces that are invisible. (Refer to step 1 in the following screenshot.)
Let's add loop cuts below the angle of each stair. This will allow us to extrude a face in the front direction of the top of each stair. (Refer to step 2 of the following screenshot.)
We will then add two edge loops at the center to add more resolution.
Now with Proportional Editing turned on (O), we can select some vertices on the stairs and move them to break the shape a little bit. (Refer to step 3 of the following screenshot.)
We will now enter the stair ramps object and delete the one on the left. We will have to delete the invisible faces, those that are in the ground and in the house.
After this, we can move the origin...