I have an office in my home used exclusively for business (bookkeeping, client contact, etc) and a very large workshop onsite but unattached to my home. Do I add the square footage of both my home and my workshop together and then figure out my office square footage or do I claim two separate home offices? My utilities and other expenses are not separated. If I did claim two different offices, how would I calculate utilities, insurance, mortgage interest etc? I'm stuck.