Where, by reason of topographical conditions or location or because of other unusual circumstances, the strict application of any provision of this part would result in exceptional practical difficulty or undue hardship upon the owner of the historic property, the commission in passing upon applications shall have power to vary or modify strict adherence to the provisions of this part, provided such variance or modification shall remain in harmony with the general purpose and intent of this part so that the historic and architectural aspects of the historic property shall be conserved. In granting variances or modifications, the commission may impose such reasonable stipulations and conditions as will, in its judgment, better fulfill the purposes of this part. The commission shall, for each variance or modification granted, place upon its records and in the notice to the applicant the reasons for its determinations.