Complex sentence: At 28 years old he made his first proposition to build on the land made available by the Foundling Hospital he was already and architect and builder in Southwark where he designed and built the Leverian Museum in Blackfriars Road.
Split into simpler sentences: At 28 years old he made his first proposition to build on the land made available by the Foundling Hospital. He was already known as architect and builder in Southwark where he designed and built the Leverian Museum in Blackfriars Road.