In an ongoing series of a cursory analysis of Boulder's new Compatible Development regulation slated to take affect January 4th, here is another set of consequences of part of that regulation - bulk planes. This study was looking at a generic south Boulder Ranch located on a north-south street. It assumes flat topography and a fairly standard size lot and existing ranch type house.
Prior to the regulations, any second floor addition was limited by the solar shadow (see the diagram below). This is not a particularly beautiful addition, but it is the simplest kind of construction - building a simple addition that mirrors the roof of the existing ranch, has an 8' high ceiling and is built as much as possible over the existing exterior walls. (new bulk planes shown in red, existing house is green, addition in white)
You can see the shadow cast to the north has limited the location of the north wall of the second story. The south gable end of this typical type of addition is in violation of the new bulk plane.
To bring this kind of addition into compliance, the south side of the second story addition would have to be move to the north by approximately 5', resulting in:
a strange little addition
To more easily comply with the bulk plane regulations, a gable-fronted second story addition could be added on:
a bit of a monster on top of the original ranch house and very different kind of style
There is an exception to the bulk plane rule that would allow a larger, second story addition that still mirrored the roof line of the ranch. It has to be at least 40' long to allow for the exception to violate the bulk plane, resulting in:
a frightening looking thing - note that a significant portion of the second story at the back of the house is hovering above the ground. The width of the allowed second story has to be 40' - deeper than the typical ranch it sits on.
In part IV we will look at the part of the new regulations that will restrict the length of the new side walls and their articulation. (In the example diagram above, the second story addition has to be 40' exactly, not an inch less or the bulk plane exception would not be allowed, nor an inch more or the wall articulation regulation would be violated!)