I’ve not been able to shed any light on to the emergence of this housing form. Web searches yield little as even the terminology seems loose. The term "Stacked Townhomes" is not reliably used to describe the form.
I don't deny that somewhere out there is the original Stacked Townhome building, the one that was actually the first time it was built. Or there may be a paper or scholarship on this, but it did not reveal itself obviously to me, so the origin and genesis of the idea will have to remain a mystery. Perhaps one of you readers will have examples that take us back closer to Stacked Townhouse Zero, and I welcome any insights or contributions that shed light on this. Till such a time we will just proceed to examine the resources that I know and that lend some insight. Let's begin.
Looking back one of the earliest examples I am aware of is not actually for the design of a Stacked Townhome, but rather for a series of high rise apartment buildings from the 1950's, designed by early modern architecture pioneer Charles-Édouard Jenerette, widely known as Le Corbusier - Corbu popularly. These apartment blocks were known as Unite de Habitation (housing unit).
Corbu's tall apartment blocks were an outgrowth of his urban design studies of the time. In these studies he advocated for large apartment blocks set into park like landscapes. Many hold him responsible for the popularization of this model, and the resulting banal cityscape that resulted. This approach has been discredited, and a great deal of the examples torn down to be replaced with more conventional neighborhoods. For the most part these failed examples were never executed with the poetry of his conceptual studies, but they begot these failed examples none the less.
So compared to his concepts, Corbu's built apartment blocks were smaller and isolated. Never the less they represented a snapshot of these larger proposal concepts. Taking the form of a narrow slab, the building ensured daylight and air for all units. Mid height there was a "street" with shops needed for daily life, a baker, a grocery, a cafe. And on the roof no luxury penthouses, but rather a daycare nursery school at the disposal of the residents. But it was in the design of the apartment units themselves where we see a kernel of the Stacked Townhome.
Instead of apartments configured as flats feeding off a common corridor on each level, the Unite were based on a two story apartment module where each apartment had one floor that fed off the corridor and one floor that spanned the full depth of the apartment block. Units were nested together such that apartments across the hall from each other were mirrored both vertically and horizontally. In each unit you enter in to the kitchen and living space, in one you go upstairs to the bedrooms and the other you go down to the bedrooms. So, similar to our Stacked Townhouse model the Unite consisted of two units, each a floor and a half nested into a three story block. The infrastructure in this case was the public corridor which in this arrangement was reduced in area by 2/3 compared to a similar block of flats as there was now only one corridor on every third floor.
The efficiency in this case came from the apartments being narrow with small corridor frontage as compared to horizontally sprawling flats. The units also benefitted from spanning entirely across the apartment block as they could now have full cross ventilation within the unit. The units were also configured with a limited two story space at the exterior wall which gave them a sense of volume and room that overcame the narrowness of the units.
This was a significant breakthrough in planning of apartment units, however it was not one that was repeated when such apartment blocks became the model for tall housing. Even today there are reasons for this. But the unit yield per corridor is 3 times that of a conventionally planned apartment building, and a similar advantage in yield is enjoyed by Stacked Townhomes when measured against the acreage and street infrastructure.
Next we'll look at some contemporary examples of Stacked Townhomes.