Under the plans submitted by the Royal High School Preservation Trust, the A-listed former school will become home to Scotland’s only independent music school, St Mary’s Music School.

City councillors agreed permission should last for seven years instead of the normal three to give the project the maximum chance of completion due to a contract with the developers behind a rival bid for the building.

Plans for a £75 million luxury hotel were narrowly rejected in December, though an appeal to the Scottish Government is scheduled to begin on November 28.

Despite the approval of the music school plans, St Mary’s currently has no right to the building because of an existing agreement between the council and hotel developers Duddingston House Properties, which it is understood may not expire until 2022.

Planning convener Ian Perry said the committee had agreed to a seven-year expiry on consent for the music school because of the unique situation with the other application being appealed.

“That will need to run its course,” he said. “It’s not unusual in the circumstances to give a developer extra time.”

Designed by Richard Murphy Architects and Simpson & Brown, the music school proposals include a performance space located in the main hall of the Hamilton building and two ancillary rooms, and a “glazed slot within the portico floor of the Hamilton building” and “glass balustrade on the terrace to the portico”.

New single-storey accommodation is also proposed for the site and demolition of the existing classroom block to the north west of the Hamilton Building and the gymnasium building to the north east of the Hamilton Building.

The project was recommended for approval by planning officials last week who said the music school would bring the building back into “long-term, sustainable future use” and would be a “significant conservation gain”.

Trust chairman William Gray Muir said: “The former Royal High School is pivotal to Edinburgh’s World Heritage status and our plans are designed to celebrate, conserve and enhance the site for the people of Edinburgh.

“By making it home to St Mary’s Music School we also hope to reinforce musical education at the heart of the city’s cultural heritage.

“We are delighted that our proposals have received unanimous approval, with the City of Edinburgh Council granting us a special, extended seven-year time-scale for planning and listed building consent.”

He added: “This is a wonderful opportunity to transform a building at risk into a dynamic asset for students, residents, visitors to the city, educational and cultural organisations and the creative sector as a whole.”

Dr Kenneth Taylor, head teacher at St Mary’s Music School, currently based in Edinburgh’s Grosvenor Crescent, added: “A move to the Royal High School under the trust’s plans would enable St Mary’s Music School to have the performance space we have always aspired to.

“It will enable us to bring music lovers into the school and greatly expand our outreach activities.”

The building at the bottom of Calton Hill was designed by Scottish architect Thomas Hamilton in 1825.