The school was unified on a single site in 1973. The Newport Road site of the former High School was eventually sold to fund an extension to Willows High School in Tremorfa, Cardiff.
The accommodation in 1973 consisted of the old Ty Celyn School Llandennis Road, Cardiff, with a new building attached, designed for six form entry. A considerable amount of internal alteration has been carried out on the original building. An extension was completed in December 2013 which added a state of the art multi purpose space; Neuadd Celyn which is used for dramatic and theatrical performances, new Music classrooms and [sound proofed] practice areas, a suite of History classrooms, Art Classrooms.
In 2014, the Sixth Form Centre [relocated] to Ty Celyn and was renovated to provide a designated centre exclusively for sixth form students. TY Celyn houses the Sixth Form Achievement Team, including Head of School, Achievement Leaders, UCAS co-ordinator and Sixth Form administrator as well as providing study facilities, recreational, and relaxation space for sixth form students.
Cardiff High School became a seven form entry school in September 1998, when a third feeder primary school, Roath Park, was added to the two existing feeder schools, Lakeside and Rhydypenau. In 2011, Marlborough Primary was added as a fourth partner school (the 'Feeder School' criterion no longer being considered for admission purposes) as the school increased to an eight form intake. As of 2016, it has a total pupil roll of 1635, of whom 450 are in the sixth form.
The school enjoys an extremely high level of parental interest and support. The establishment of a new Cardiff High Partnership with parents in 1998 both built on the strong, existing Parents' Association links, and launched new initiatives, including a covenant scheme. It also expanded the range of educational, social and fundraising activities.
