Side Yard Bulk Planes As part of the recently passed Compatible Development regulations, the City of Boulder has included a bulk plane restriction. This regulation limits any new construction or addition from building beyond a projected line starting 12' above the property lines and sloping inward at 45 degrees.
As a real-world application, the impacts to possible second-story additions are significantly more restrictive than those imposed by the previous set of regulations. As mentioned in the first post about these regulations, the devil is in the details, or more specifically in the combination of the new regulations with the existing solar shadow ordinance.
For so many homes in the affected areas, one side of the house is within 5' of the property line in accord with the setback regulations. This means any possible second story addition would be limited similar to the above diagram. The available interior space of this addition is significantly lower than previously allowed as the head height at the edge of the room is now under 6'.
Under the older regulations, Boulder pop-tops tended to be partial extensions of the floor below, weighted to the south side of the property to avoid the solar shadow restrictions. As in the photo above, the addition to the original ranch created a distinctly either right-sided or left-sided house depending on what side of the street the property was located (again, driven by shifting this mass to the south side of the property).
The new bulk plane regulations will either shift this kind of the addition down, making it more like a series of dormers sticking out of a new roof, or shift the south side of the addition closer to the center creating a kind of bubble addition sitting on a ranch house. There are some exceptions allowed for penetrating the bulk plane, however, most of the existing houses are not of a size to take advantage of some of the them.
So, the regulations may encourage more wedding-cake style houses with reducing layers as the building moves upward.
The former regulations lent to additions that created a kind of asymmetrical balance - playing the long, low horizontal lines of the ranch house against the vertical lift of the addition. Weighted to one side, this composition often closely paralleled the plan of the house, with public rooms of living room, dining room, kitchen, etc. on one side of the house and bedrooms on the other.
The wedding-cake style of house creates an entirely more complex composition, being neither predominantly horizontal or vertical. The local architects, and especially builder/designers, will need to work harder to make each house or addition sympathetic to its neighbors and architecturally worthy.