Expenses that can be traced directly to the business can be recorded on your books as is. For example, if you repaint your office, that's a pure business expense. If you get a rider on your homeowner's insurance policy covering your home office and business equipment, the full cost of that rider is recorded by the company. When an expense crosses the line between business and personal, such as a security system that covers the whole house, you'll have to figure out what portion of that expense belongs to the business (see the next section for how to do that).