1. What do you need to do your job?
Not all home offices are created equal. A computer and small desk might not cut it for you. Sure, if you're building a home office just to check your email and pay your bills, that's all you'll need. But an architect who might want to bring his work home is going to need plenty of drafting space and natural light. Think about not only what you'll do in your home office now, but also what you'd like to do in there in the future. Girlfriend moving in soon? Make room for two. Thinking about expanding your Etsy store? Make sure you've got the room for a sewing setup.
2. Do you need any privacy or security?
Privacy is more than a closed door or computer password. If your work handles financial or other confidential records—including your own—make sure you've got at least one locked, fire-safe cabinet.
3. Is there adequate wiring?
Converting a closet into an office is a great idea—until you realize that there's not usually any outlets in closets. No matter where you're getting ready to build your workspace, check it out for outlets and phone or cable jacks. If there's not enough, don't scrap your project. You can always get an electrician to add extra power outlets and use a wireless connection for internet.