An irregular perimeter form is applied to any building plan which does not closely approximate any other defined perimeter form. Most examples are unusually formed, or combine elements of multiple forms (such as a semicircle conjoined to a rectangle). This form is only assigned if the general impression formed by the outline cannot readily be assigned to another form. Small irregularities or unevennesses are not sufficient to make a generally regular shape irregular.

The whole outline is assessed before an irregular form is assigned. A combination of otherwise familiar shapes can qualify when no single defined form describes its general impression.

Small notches, uneven sides, or chamfered corners are expressly allowed in several regular-form definitions. Those details alone do not make a rectangular or other recognizable plan irregular.