Issuance of certificate. Within a reasonable time after application is filed, or within such further time as the applicant may in writing allow, the Historical Landmarks Preservation Commission shall take final action to approve or deny the application for a certificate of approval. Such application for a certificate of approval shall not be approved unless the proposed construction, reconstruction or alteration of the exterior architectural feature involved will be appropriate to the preservation of the historical landmark for the purposes of this article or unless, notwithstanding that it may be inappropriate, owing to conditions especially affecting the structure involved, failure to issue a certificate of approval will involve a substantial hardship to the applicant and such certificate may be issued without substantial detriment to the public welfare and without substantial deviation from the intent and purposes of this article. In passing upon appropriateness, the Commission shall consider, in addition to any other pertinent factors, the historical and architectural value and significance, architectural style, general design, arrangement, texture, material and color of the exterior architectural feature involved and the relationship thereof to the exterior architectural features of other structures in the immediate neighborhood.