Section 9. Nothing in this chapter shall be construed to prevent the ordinary maintenance, repair or replacement of any exterior architectural feature within an historic district which does not involve a change in design, material, color or the outward appearance thereof, nor to prevent landscaping with plants, trees or shrubs, nor construed to prevent the meeting of requirements certified by a duly authorized public officer to be necessary for public safety because of an unsafe or dangerous condition, nor construed to prevent any construction or alteration under a permit duly issued prior to the effective date of the applicable historic district ordinance or by-law.