He decided to leave in 1920 and started his own architecture design in Brisbane where he practiced for his entire life.
His domestic work included houses such built in the Ascot, Hamilton, Clayfield, Coorparoo and Greenslopes.
He designed houses in a range of styles, including the Californian bungalow Gibson residence in Manly (1919).
The Raff residence in Clayfield (1923) and his own house at Greenslopes (1926) are good examples of designs presenting those features.
Cassa Anna (later El Nido) in Hamilton (1926) and Santa Barbara in New Farm (1929)[3] are two examples of Trewern working in the Spanish Mission style.