Law & Taxation mentor Barbara Weltman responds:
As long as your sideline business is the principal place of business for that activity and you use a portion of your home regularly and exclusively for business, you may qualify for a deduction. You do not have to work full-time from your home to be entitled to a home office deduction. To learn more about the requirements for claiming a home office see IRS Publication 587, Business Use of Your Home, which you can download from the IRS Web site.