I used to live on the upper floor of a two-story “garden apartment” (which is a uniquely North American style of building that was popular around the '50s-'70s, as far as I can tell) that was designed basically like groups of adjacent quad-plexes. I had a front door from my living room to an interior staircase that opened out onto the complex’s courtyard, as well as a back door from my kitchen to an exterior metal staircases that went directly to the parking lot. There were no common hallways and each unit had windows on two sides (the front and back).

In terms of the staircases, at least, the design was very efficient.

The only limiting factor stopping them from building more stories on top with the same design (other than that tenants would start wanting an elevator) is the fact that the site basically maxed out the area available for a surface parking lot. Building more stories would’ve required also building a parking deck in order to meet zoning requirements, even though the parking it already had was never more than half-full.