You actually have a decent number of ways you can qualify a freestanding structure for a home office deduction. It doesn't necessarily have to be a computer-printer-copier-packed studio with a neon "OFFICE" sign over the door. Workshops, garages, barns -- any structure you use exclusively for work counts. You don't even have to meet clients there, and it doesn't have to be your principle place of business.
The example the IRS gives is so pleasant that it deserves to be repeated (albeit with my own idyllic embellishments). Say you run the cutest little floral shop ever on Main Street. If you have a greenhouse in your backyard that you use to grow your flowers and plants for the shop, you can write off some of its (qualifying) expenses as deductions. It doesn't matter that you have a place of work on Main Street; you're using that greenhouse solely for your business [source: IRS]. So not only do you get to have a lovely job where you nurture plants and flowers all day, but you get a tax break for your super cool greenhouse, too. Lucky you!