The short answer: they already did both.
The long answer: Generally, libraries use a 20-year planning horizon, as set forth in Serving Our Public: Standards for Illinois Libraries. Most building bonds are 20 year duration and that's about how often libraries propose referenda to their constituency. Despite outstripping all population projections for the Library District's service area by 2003, the Library's Board of Trustees chose to wait while the school and fire districts ran referenda to ensure they could keep up with the demand for service. They examined the existing building and made renovations in 2004 within it to get by for several more years.
From the beginning of the expansion planning process in 2006, the Library Board targeted 2010, the year the 1990 building bonds are paid off and the Library District is contracted to purchase a piece of property owned by the Village of Plainfield adjacent to the downtown Library. One of the first activities of the planning process was development of a Space Needs Analysis, based on population projections and Illinois library standards.
When the Space Needs Analysis showed a need of 135,000 square feet of library space to serve a population of over 124,000 by 2030, the Library Board considered both the potential impact to the taxpayers and the fulfillment of the need for library service. They chose to scale back the project to 100,000 square feet of library space that would serve a population of up to 95,000. Anticipating 2 years from groundbreaking to opening day, a significant wait is included in this timeline even if the referendum passes in April.
The Library Board waited from 2003, when the Library was only 35% too small, until 2009, when the building is now 60% too small according to Illinois library standards and local demand for service. The Library Board scaled back from the 20-year planning standard and 135,000 square feet of space to a 12-15 year planning horizon and 100,000 square feet of space.