World's oldest architectural practices continuously operating

This article lists the oldest extant architectural practices in the world. Very few architectural practices are still operating whose foundation pre-dates the beginning of the 20th C and it would not be unreasonable to conclude that any practice that was founded in the 18th or 19th C should be listed as amongst the oldest surviving architectural firms around the globe. In order to qualify for the list a practice must:
 * be predominantly an architectural practice, designing and constructing buildings and like structures
 * be registered in their home territory as architects or architectural designers
 * be operating continuously from the date of foundation, albeit a change of names or ownership is likely over intervening centuries
 * be evidenced by relevant authoritative sources