Solutions for Small Spaces
You may have a finite amount of square footage for your office, no matter how creatively you look for space. In this case, consider separating your records storage from your office area, perhaps putting a filing system in the attic or basement (first solving any heat or dampness problems that could damage the stored items). Off-site storage for records that you don't need to access frequently might also be an option.
In terms of making the best use of the space that you have, try multifunction furniture. You can find computer desks, for example, that resemble armoires when the doors are closed. Open up their doors, and shelves flip down to create work surfaces, filing drawers roll out, and the computer's ready to go, with a bulletin board space on the back wall of the desk.
You can also make use of awkward space in your home for bookshelves and storage areas. Look at the spaces under stairs, for example. Can you fit a small desk and a set of wall shelves into an unused alcove on a stair landing perhaps?