Complex sentence: Heald (2012), page 123 The original house was completed in the mid- 1860s by an unknown architect but was transformed by architect Richard Norman Shaw between 1869 and 1884 into an imposing mansion.
Split into simpler sentences: Heald (2012), page 123 The original house was completed in the mid- 1860s by an unknown architect. This was the genesis of the transformation of the house between between 1869 and 1884 into an imposing mansion.