Complex sentence: The growth of these elements in the late 18th and 19th centuries, led to an expansion in the proportion of buildings which are of polite design which has continued into the 20th and 21st centuries.
Split into simpler sentences: The growth of these elements in the late 18th and 19th centuries, led to an expansion in the proportion of buildings which are of polite design. This trend has continued into the 20th and 21st centuries.