North Yorkshire County Council planners green-lit plans for the two-storey facility and demolition of the old sixth form building earlier this month.
Pending a tending process the building could be built in time for the new academic year in 2019.
Headteacher Carl Sugden said: “We are delighted that we have the support from North Yorkshire County Council in improving our sixth form provision. The development of this project has been a long time in the planning and will replace an existing facility which is no longer fit for purpose.
“King James’s Sixth Form has performed consistently in the top 25 per cent nationally for the last ten years and this is a great opportunity to put our high performing sixth form into 21st century facilities.”
“The new build also supports our capacity to re-configure our internal space as housing growth in the area continues to increase pupil numbers.
“The sixth form is also growing and this will improve our study facilities, café and learning spaces.”
Plans show the building will be constructed behind the existing sixth form block, demolition work will be carried out once this is completed.
The old sixth form is to be replaced by a bus park which the school says will improve traffic management.
Harrogate Borough Council dismissed plans when consultation documents were put to them in August last year, flagging heritage concerns.
However the county council ruled that the cost and age of the building mean it should be replaced.
A spokesperson for NYCC said: “The existing sixth form building has now reached the end of its life.
“The building needs to be re-roofed, which would be prohibitively costly and poor value for money. The overall age and condition as well as its layout does not provide suitable accommodation.”