This was the first building I started on when I decided to make 25mm scale terrain for FMAS. I had the general idea of building a house, but ended up adding details as I went along.
The walls are made out of corrugated cardboard. I made a template for the windows and door and drew the positions on the cardboard before cutting anything out. That way the windows would be the same size and height all around. I used masking tape to cover the edges of the windows and the corners so the corrugation wouldn't show.
I decided to add some battle damage to the rear of the house. First by creating a broken looking doorway, and then adding a boarded-up window using flat toothpicks. I also added a few knicks and dents in the walls.
After contemplating various methods for making a roof, I decided to attempt shingles. I cut cardboard from a cereal box in to half-inch strips. Then I notched the strips along the long side nearly all the way through to make the shingles. I compared each new strip with the previous so the shingles would overlap. Finally I glued the strips together so that the top one overlapped the uncut portion of the strip below it. I made each side of the roof separately and then glued them both to pieces of corrugated cardboard strength. The cardboard had already been glued together at the correct pitch for the roof. I covered the ridge of the roof with individual pieces of cardboard to finish it off.
The walls were then covered with a plaster/water/glue mixture to give them texture (and to hide the masking tape). The base coat was almond spray paint (I ran out of black), the walls were painted brick red, and the boards covering the window were painted with nutmeg brown.
I made the window glass out of plastic from a milk carton. They give an sort-of-glass look and are opaque to cover the empty inside.