If you want to add a room or change the function of a room, especially if that includes running plumbing or additional wiring, be sure to do this prior to starting production. It is always easier and less costly to build it originally rather than to renovate.
Modular home companies will offer custom paint, flooring and added options, but talking up front about what you want is crucial. If you have samples, colors or fabric swatches you want to match bring them in and share them with the architect or designer.