Operating expenses are any expenses that are incurred for the upkeep of a rental income. Some common expenses include maintenance, taxes and insurance. Hiring a rental management company to oversee your properties count as well. If the washing machine breaks, that would be considered an operating expense. Painting the outside of the building would be considered an expense as well. Anything that will cost money while eating away at your profit is an operating expense. Purchasing equipment for a fitness facility would be another operating expense. The lights, water and cable to keep that facility running and engaging for tenants would also qualify. It should be noted that making a mortgage payment is not considered an operating expense. However, you may be able to deduct it as a business expense for tax purposes.