I’ve been playing around with SketchUp, it’s a cool, easy to use 3D modeling program. Google bought it late last year and made it freely available. (Note: Google, please buy Elsevier next.) SketchUp has a similar feel to the SIMS games. You can design interior or exterior environments. There are also lots of free bonus materials, like chairs, electronics, carpet patterns, HVAC systems, people, appliances, etc or you could create your own materials and components, such as shelves and tables that are specific dimensions.
You could quickly create a rough sketch of your library and then tinker around with different designs. This is ideal for renovations because you could see just how much stuff you can fit in a space and what it will look like to patrons. It looks a lot better than simple CAD drawings– easier to sell people on the idea.