Jersey's new school will cost five million pounds more than was originally planned.
The new building for Les Quennevais School will cost £45 million.
The increase in the overall cost is blamed on delays and changes to the design of the site.
The project was approved this week after initial plans were rejected earlier this year over concerns with parking and layout.
It will be the biggest single investment in education in Jersey's history and the first new secondary school in more than a decade.
The target date for the first lessons in the new school is September 2020 and, when finished, it will accommodate around 800 students.