The House is the key organisational unit in the School. Over 300 students board full-time at Wanganui Collegiate School. Each House has a mix of Day and Boarding students. The Day students share all facilities and have their own study spaces. It is common for day students to take part in the supervised study times in the evenings and to stay overnight.
Houses compete in activities that range from music and debating through to netball and cricket.
The girls' Houses, however, compete strongly in their own right in some competitions. Students in boarding houses are in the care of a resident Housemaster, assisted by a resident Assistant Housemaster. The resident House Matron supervises domestic arrangements and provides pastoral support for the students in her care. Year 9 students share living space with one or two others. Prep is one of the major advantages of boarding life.