Complex sentence: Most of the rowhouses were built in the 1870s and 1880s, probably by the same builder, as they are a more cohesive group than other clusters of rowhouses in Park South.
Split into simpler sentences: Most of the rowhouses were built in the 1870s and 1880s by (it is believed) the same builder. They are thus a more cohesive group than other clusters of rowhouses in Park South.