It can be difficult to convert an existing non-library building, or even to renovate an older library, for structural reasons. Public libraries require a load capacity of 150 psf (pounds per square foot) throughout the building, and 300 psf for any areas with compact shelving. By contrast, standard office buildings are built for a load of 50 psf. Most non-library buildings are not capable of handling the weight of collections, and beefing up the structure can be expensive.
There is nothing intrinsically wrong with the library continuing to inhabit a historical monument providing there is adequate funding to complete restoration work and to provide the library additional space elsewhere, if necessary. Restoration work is often expensive and operating costs in many historical buildings are higher than they are in contemporary structures of equivalent size. Some academic institutions and governmental agencies are unwilling to accept the higher costs of renovation, operation and maintenance. In such instances, the librarian should carefully separate the provision of library materials and services from the age of the building it occupies: the library is not the building, it is the materials, services, staff and users that the building shelters. As simple as this may sound, it is a point missed by many who would commit the library an eternity in an inadequate home simply because of a long association of one building with the library.
When public sentiment runs high over the historic monument syndrome, the librarian must avoid becoming entangled in this emotional web. This can be done by understanding the underlying causes of the controversy while concentrating on a careful analysis of the alternatives. Wherever there is reason to believe that the historical monument question will arise, it is imperative that the librarian encourage a careful and objective investigation of the alternatives before it becomes a public issue.
--From Planning Library Buildings and Facilities: From Concept to Completion (1989), p. 41, by Raymond M. Holt
Should you renovate or add on to your existing facility? Should you tear it down and start over? Is a completely new site the best way to go? Your architect will be instrumental in providing technical information and recommendations.
This document from the Libris Design group can provide background material to help you with the decision-making process. Keep in mind that the case study given is in California and is not a designated historic building; however, the process will work for any library.