When the perimeter shape is set to 1-Polygon, you can set the perimeter Line type to one of:
When the line type is set to 1-Straight line, a perimeter is drawn by placing the origin and then a series of line end-points:
To incorporate an arc within a perimeter, set the line type to 2-Arc. An arc is defined using the sweep angle and number of segments:
A negative sweep angle can be used to create an arc in an anti-clockwise direction. In DesignBuilder geometry, arcs are created using a number of segments or straight lines and the accuracy of the representation can be controlled by defining the number of segments. Generally speaking, the greater the number of segments used will result in more memory usage and processing time and so, particularly in the case of blocks, it's advisable to use the default number of segments where possible.
To draw the arc, place the arc centre:
More information on using the Arc line type can be found in the Basic Drawing Tools 9 Tutorial