The is to undergo £2.1 million conversion just two years after it opened to pave the way for a language school to move in.
The which was announced earlier this year, will see the £188 million library open to the public for longer following a dramatic cut in its hours, from 73 down to 40.
But it means the first floor conference rooms and study areas will be converted into 24 classrooms for the language school.
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The £2.1 million cost will be borrowed by the city council and the debt repaid when the existing Brasshouse site near Brindleyplace is sold.
Labour cabinet member for culture Penny Holbrook said the conference facilities had not proved as popular as initially hoped and study areas would be found elsewhere in the building.
She said: "There are two important things here. We have a world-class language service moving into a world class building.
"This allows us to keep the library open longer than it has been. The cost of £2.1 million is expensive but it is an expensive and beautiful building and we want this done properly.
"This move is a fantastic opportunity for the library and for the Brasshouse, whose users will benefit from a more modern environment with an even greater range of resources."
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Work begins early next year and it is hoped the conversion will be complete and the Brasshouse open from September 2016.
The move means the library will be open to the public from 9am to 9pm weekdays, including an express book borrowing service.
The specialist research and archive facilities will still have limited opening times.
Earlier this year, the and 11am to 5pm Wednesday to Saturdays as a result of budget cuts.
The and students.