Complex sentence: He returned to his father's firm to work as his assistant and designed his first realized building in 1884, pension Oud Leyerhoven, a house in the Vondelstraat in Amsterdam, a street planned by his father and otherwise mostly filled with houses to his design.
Split into simpler sentences: He returned to his father's firm to work as his assistant and also started designing building under his own name. His first realized building was built in 1884 and was pension Oud Leyerhoven, a house in the Vondelstraat in Amsterdam, a street planned by his father and otherwise mostly filled with houses to his design.