I would think that the whole left side of the house with the garage and upper balcony is the addition. They did not build houses like this in 1955.
I don't think so. The house appears to have been built as a four level split.
Split-Level
"An example of a split-level residence in the Denver area. Please click on the image to see a larger version.Often referred to as a tri-level, a split-level is more of a building type than a style. Developed in the 1930s, it emerged in the 1950s as a multi-story counterpart to the dominant one-story Ranch house. Retaining the low pitched roof, overhanging eaves and horizontal lines of the Ranch, these homes added a two-story unit connected at mid-height to a one story section creating three staggered floor levels.
http://www.historycolorado.org/oahp/split-level
In a city where there is permafrost, a four level split would make more sense than a three level split, as there would need to be a four foot basement ... so why not make it 8 feet with useful living space.