The project is being partly paid for by the Government’s Selective School Expansion Fund and a loan.
In addition school bosses have launched a fundraising appeal for a further £75,347 to furnish the new building.
Construction of four additional maths classrooms, a new dining hall, kitchen, science laboratory and an ICT suite is scheduled to start on January 4.
Newport Girls’ High’s head Michael Scott said on its website: “All my colleagues want to provide the very best education we can to our students and the new classrooms, laboratory, computer suite and drama studio will enhance the curriculum and offer even greater opportunities to all students.
“We know that launching a fundraiser in the midst of a pandemic is far from ideal, but we cannot stand still as a school and we will have fantastic new facilities in just over 12 months’ time.
“Hopefully with the support of parents, friends, governors and alumnae, we will have the right furniture and fittings inside the building’s shell.
"Thank you, in advance, to all those who are able to contribute to making our vision a reality.”
The building project follows a change in admission policy at the Wellington Road school to include 15 places for children who receive pupil premium support.