Theres multiple ways of stacking buildings.
For example... heres 3 of them.
One is lightswitch stacking... used alot in defenses.
Lights off with a hotkeyed probe or SCV... and build. Many of you are familiar with this.
This works with Terran and Protoss. (you can also have the Buildings that can lift off and all that jazz... They can Build addons in the dark, or land in the dark.)
The other is Pre-Place stacking. a few Stacked Sunken Defenses are still "stackable"... but to a limit.
Basically the sunks are pre placed, and all you do is have an ultralisk kill its way through, making the path... and theres "pre placed" sunkens that are "stacked". All of you are familiar with this.
There is ONE more way to have Zerg stackable... with the lights.... I remember doing this for one of my old sunken D maps that got killed by the 1.14 patch.
This method requires that the player commands a Liftable Building (CC or Barracks.. Factory... Starport... etc.)
There also needs to be a lightswitch.
And... the Zerg race with Drones.
Basically, you start with the lights on.
Then, you have the drones build the first layer....
Then , have the building (Hotkeyed as 2) land ontop of the sunks while the lights are off.
Turn the lights back on, and lift off the building. You can have drones stack wherever the building was.
There are very very slight alternatives to this... you can have a probe build pylons, and then just kill off the pylons after you turn on the lights.
Hope i helped!
None.