dont want to bog this down with too many CEC references, but for a garage, there is a 10watts per square meter value from table 14, and based on 8-210(b) the loads described are "special loads" and as such the rating of the equipment must be used for service ampacity with demand factors permitted in other sections of the CEC. In other words, 150 amp service could be okay, depends on the loads on the sub panel,the square footage of the garage, and the nature of this new 230(?)volt 50amp breaker. Most likely a 200 amp service will be required, but that means new feeders, possibly existing conduit size could be a concern.
