The last part of the upgrade to Wade House, home of the Deepings Community Library, begins on Monday.
The work, to install a disabled access public toilet in the library, is being undertaken by Lincolnshire County Council, as part of the refurbishment of the building.
It will be finished by Easter at the latest.
During the work, part of the library will be shut off and this will mean that the junior and adult libraries will be sharing the same space for a while.
The rear door will also be out of bounds, so users will need to use the front door. If anyone cannot manage the steps, disabled access through the staff entrance will be available.
A spokesman said: “The Friends of Deeping Library are delighted that this amenity is being added to the library, which will remain open and all our activities will continue throughout the work.
“We hope users will forgive any temporary inconvenience it may cause.”