One of the things that I enjoy most about living in the District is exploring new neighborhoods with my dogs and always finding something new to appreciate. While I enjoy the finer details apparent in individual homes, such as cornices and brightly-colored doors, I can’t help but notice when something seems out of place on a larger scale, like a home set closer to the street than the rest on a block, or one home that appears to be taller than the rest. The latter is referred to as a “pop-up,” which can be visually disruptive to neighborhood character. 
What are pop-ups? Pop-ups are spaces that are constructed above the ceiling height of a dwelling. Sometimes, they are used as attics or storage space, but often they are converted into habitable space, such as a small room or additional story. Dwellings with pop-ups are generally taller than surrounding homes, which disrupts what would otherwise be a consistent roofline in a neighborhood block.
The Office of Planning has received a lot of feedback concerning pop-ups, and the general consensus is that if inappropriately done, they can alter the character of the neighborhood. Pop-ups can cause variations in height that are found to be inconsistent with the architectural and historical design of a neighborhood. So, OP is addressing these concerns in the proposed zoning regulations.