It seems you are arguing that 1x1 should be split into 1x1 exterior and 1x1 interior.
i am suggesting they are already so divided, that the 1x1 interior is just absent, and that the authors that are using 1x1 exterior plot template for an interior plot are already coloring outside the lines in an effort to fill this conceptual gap and create the artistic vision they have in their head. the only 1x1 plan in the base game is external only (i believe this is your
"protect and serve" advanced defense plot, right?)
there is a distinction to be drawn between interior plots and exterior plots, and a distinction to be drawn between plot sizes, and having the current T-shaped size chart leaves holes for 1x1 interior and 3x? interior templates is creating a technical debt for downstream authors of plots that is starting to show through at the seams. once again, i feel the 3x? plot size is probably ignorable; large interior spaces are sub-dividable to provide for multiple smaller plots, but the reverse isn't true and the 1x1 interior size is needed.
There's also a fairly easy way to communicate what you've just stated to the player or the settlement designer - it's in the building plan description.
the point that the mayor would ever see the building plan description, the plot template has already failed to do it's job of filtering a reasonable default plan for the plot to the end users satisfaction. manually picking a plan, or even a class of buildings, is a failure state for some automation focused players, and i find myself on the borderline of that group.
i know how much effort authors, and you particularly, put into the plan descriptions, and for people who are fine-tuning a city plan, or carefully selecting plots for their vision, or are passionate about building plan style and model looks, that's great value. the same could be said for style tags and theming, which is great for one group of passionate creators to communicate to another group of sophisticated consumers.
for the non-passionate players who want a reasonable default answer to the question of storing these settler corpses in this vanilla building space so they can move on to solving other problems, the building plan description is too little too late. a municipal plot where i know i have to manually pick a plot a class is a minor but predictable failure of this automation, but having a residential plot pick interior plan for an exterior plot or vice versa is the worst case scenario because not only do i have to do rework on the plot, but it adds a bunch of additional work filtering plans that isn't otherwise required.
as a consumer of plan packs, i run into this problem with almost all of the 1x1 plots that are interior-designed, but are selected and built on exterior 1x1 plots unless the i carefully and manually selects plans. nobody wants a well apointed marble bathtub under the stars or an outhouse in their prewar home.
i'd argue this is also a lesser problem with building plans that assume some foundation, such as water-based and earthen-works plans, such as
Wasteland Venture's Baron's cemetery or your own
Float On tarberry bog. both of these plots, and many others, violate the conceptual expectation that the space inside the plot and the space outside the plot are modular and disconnected. getting either of these plots as the default plan selected by the framework on the 4th floor at hangman's is a failure state of the framework, not the author or the player.
a case i didn't think about at the time is
this alignment problem, where exterior and interior plots have different vertical sizes, which can't be resolved by anything in the building plan.