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1. In the districts indicated, for those buildings that are subject to Section 23-421 (Basic pitched-roof envelopes for certain districts), permitted obstructions are limited to chimneys, dormers, qualifying exterior wall thickness, flagpoles or aerials, parapet walls, roof thickness, skylights, solar energy systems and vegetated roofs pursuant to Section 23-411 (General permitted obstructions).
2. In the districts indicated, for buildings other than those subject to the provisions set forth in Section 23-421, for those buildings subject to any height and setback regulations, the following rules shall apply within a required front setback distance above a maximum base height:
   1. Dormers shall be allowed as a permitted obstruction, provided that on any street frontage, either one of the following methods is applied:
      1. the aggregate width of all dormers at the maximum base height does not exceed 60 percent of the width of the street wall of the highest storyentirely below the maximum base height. For each foot above the maximum base height, the aggregate width of all dormers shall be decreased by one percent of the street wall width of the highest story entirely below the maximum base height; or
      2. the aggregate width of all dormers at the maximum base height does not exceed 40 percent of the width of the street wall of the highest story entirely below the maximum base height. Such dormers need not decrease in width as the height above the maximum base height increases.
         
         Where towers allowances are utilized pursuant to the provisions of Section 23-435, dormers may only rise to a height equivalent to 75 percent of the height of the tower portion. Such dormer shall be included in the calculation of tower coverage.
   2. Solar energy systems on a roof shall be limited to four feet or less in height, as measured from the maximum height limit, or the finished level of the roof, whichever is higher. However, on a roof with a slope greater than 20 degrees, such systems shall be limited to 18 inches in height as measured perpendicular to the roof surface.
   3. Wind energy systems shall not be allowed as permitted obstructions.
   4. Window washing equipment shall not be allowed as permitted obstructions.