The Library of Birmingham is to open for 12 hours a day as part of a shake-up which will see the Brasshouse Language Centre move to the £188 million building.
The library will be open from 9am to 9pm on weekdays from early next year ahead of the world-renowned centre moving from its current base in Sheepcote Street next to Brindleyplace.
The announcement eases pressure on Birmingham City Council after an angry reaction to reducing opening hours from 73 per week to just 40 as part of wider cost-cutting.
However, it will remain closed on Sundays and operate with a reduced staff in the evenings, with some self-service.
The move also secures the future of the Brasshouse centre, which caters for 7,000 students.
Coun Penny Holbrook, cabinet member for skills, learning and culture, said about 20 classrooms and break-out spaces would be created and none of the services at the country's largest adult education centre would be lost.
She said: "I think it is brilliant to be able to bring a world class language service into a world class library.
"It is not about shoe-horning two things together – it is about improving them by co-locating. It also allows us to open the library service."
"We have had a look at how people use the library," she added.
"The core library function will be available 40 hours a week but the building - librarians, the café, wi-fi and so on, will be available through the evening."
The Brasshouse, which currently offers training in 30 languages, will operate as Brasshouse @ Library of Birmingham following the move in September 2016.
The tie-up is part of a wider push to get better value out of the library, which is costing the authority upwards of £20 million a year, including a large chunk of interest costs.
Presently, the library opens at 11am and closes between 5pm and 7pm from Monday to Saturday and is closed all day Sunday.
Those hours will remain the "core" operation, with reference and archive staff in situ, but access to books, wi-fi and cafés will be extended by 26 hours a week.
Coun Holbrook added: "We have listened to feedback from residents and library campaigners and recognise people want to use the library for longer and for a variety of uses.
"People don't just want to use the library for borrowing books and reference purposes but want to use it as a study space, to work and research, to access the internet, to visit as tourists and to simply hang out and relax.
"We always said we would look at all options available to us when it came to keeping the library open for as many hours as possible, though of course these things do take time to arrange and we had to ensure we get it right."
A business case will be drawn up about how the Brasshouse will fit into the second floor, which is presently largely used for conferencing.
The council is expected to work alongside its original architects and the building in Sheepcote Street is due to go on the market shortly after the move.
However, Coun Holbrook said the complete closure on Sundays, which was announced in April, would continue for the foreseeable future.
She said: "The reality is that opening on a Sunday is a lot more expensive than across the week, so it would mean being open a lot less during the week.
"Virtually none of the major libraries across Europe are open on a Sunday and we have to be realistic about what we can achieve. Most of the complaints about opening hours have been about access, so hopefully this will be received well."