The project, designed by Rick Mather Architects, involves refurbishment of the east wing of the college's main building.
Construction company Stepnell, which has an office in Wantage, is working on the major revamp at Grade II* listed Mansfield College in Mansfield Road.
The work is due to be completed next February. Stepnell managing director Mark Wakeford said: "Our team is excited to be working on this major scheme, which will bring real improvements for students, staff and visitors. "We have a strong track record of working in historically sensitive settings, experience that will help ensure the smooth delivery of this important project." The extension features an atrium with a frameless glass roof, and will provide access to all parts of the east wing. It will also house new kitchens. On the refurbished ground floor, a new student refectory and café bar is being created with the remodelling of the Victorian kitchen. Two stone-framed windows will be converted to glass doors to a new south-facing sunken terrace, opening on to the historic college quad.
The scheme is doubling the catering capacity of the college as well as improving its conference, fine dining and corporate facilities. A new stairway and lift will link the extension to the rest of the college. Stepnell will also refurbish existing tutor rooms, and the scheme includes the creation of a new workshop. Colin Cook, Oxford City Council's executive member for planning, said the scheme was granted planning permission about 18 months ago.
He added: "This is a sensitive intervention on a constrained site. "Rick Mather Architects further cement their reputation for finding architectural solutions to the 21st century demands being placed on historically important heritage buildings." No-one from Mansfield College was available for comment.
London-based Rick Mather Architects completed the award-winning £61m extension to the Ashmolean Museum in Beaumont Street in 2009.