Density bonus. Upon a showing of exceptional design and/or public benefits, the Planning Board may, by a special permit, authorize a density bonus of up to 20% of the total number of dwelling units permitted in a project. Fractions of units shall be rounded up to the nearest round number. The affordability requirements of this bylaw do not apply to density bonus units. The Planning Board shall consider factors such as consistency with the Chelmsford Affordable Housing Plan, dated December 2011, or other planning documents, unit type, population served, site design amenities and public benefits that integrate the project into the neighborhood in which it is located. Density bonus units as allowed by this section shall not be applied in addition to density bonus units permitted by other sections of the bylaw.