Timaru Boys' High School has always had boarding pupils. Early in 1880, the first Rector, Mr Dawson, advertised that he was "prepared to receive into his house, on Otipua Road, a limited number of boys attending the High School". The practice of taking in boarders was continued by Mr Hogben who, for this purpose, leased a house in Craigie Avenue.
Not until 1907 was a hostel as such built at a cost of 2,312 pounds on the North Street site that had been purchased the previous year. The choice of the title, Rectory, for the new hostel was singularly apt for it was at that time the home of the Rector and his family and was administered by him as a private enterprise independent of the Board. In 1918, when the house system was introduced, the Rectory became known as School House. The hostel was conducted as a personal business venture on the part of the Rector, Mr Thomas, until his retirement in 1935 when the Board took over control of the hostel and renamed it Thomas House in Mr Thomas's honour.
Fraser Wing (1962) is named after Mr Hanson H Fraser MBE (1881-1959) a member of the Timaru High School Board from 1923-1959 and Chairman of the Board of Governors from 1936-1959. It was totally redeveloped over the period 1987-90.
Jubilee Wing (1984) is named after the 75th Jubilee of Thomas House, the proceeds of which met the cost of this building. Jubilee Wing was totally redeveloped in 1999.
Manning Wing, purchased in 1997, is named in recognition of the Manning family (Robert who was a pupil (1941-45) and member of the school and hostel staff for over 25 years, his wife Betty - hostel matron, and his father, Archibald, member of the Board (1942-65).
The Ministry of Education handed over Oxford Wing, the caretaker's former house, to the hostel in 1999 for senior students' accommodation. Raymond Wing was purchased in 2002 for housemaster accommodation. The original prep school is used as a weights and games room. In 2005, Lindsay Wing, consisting of three motel units, was built to accommodate 12 seniors in a flatting situation. Lindsay Wing is named for David Powell Lindsay (1920-24), the school's first Olympian, and his cousin, David Frederick Lindsay (1922-25) head of Thomas House and an All Black.
From the original eight boarders in 1908, the hostel rolls have fluctuated markedly, rising to above 200 in the 1960s. In 1982, following major renovations and rebuilding, hostel numbers were set at 75. Thomas House currently caters for up to 125 boarders.