In the Income Approach, the deductions from gross income are typical and reasonable operating expenses, and taxes are considered typical and reasonable expense items. However, for tax assessment purposes, the taxes are not known. The values are being established so that the tax rate can be calculated for the current year. If the assessor is establishing a value for 2013 and is doing it at the beginning of the year, the tax rate is typically not known yet. If they are to include a tax component in the Income Approach as an operating expense, then they would have to use the prior year’s tax amount. Because the Income Approach and resulting value is going to have an impact on the current year’s tax rate, last year’s tax would affect the new tax rate. As a result there is a circular argument against using last year’s tax in the Income Approach.

Instead of including property tax as an expense item, the tax assessors add their effective tax rate to the appropriate capitalization rate for a particular property type in a particular market area. This gives a property tax component influence on the final value, but it’s not used as an operating expense and it’s not used as an actual number, such as the prior year’s tax amount. The loaded capitalization rate is then applied to all net income produced by the property, which, in turn, produces a value estimate.

Net operating income (NOI) is defined by the Appraisal Institute as "the actual or anticipated net income remaining after all operating expenses are deducted from from effective gross income, but before mortgage debt service and book depreciation are deducted."

Effective gross income (EGI) is "the anticipated income from all operations of the real property adjusted for vacancy and collection losses. This adjustment covers losses incurred due to unoccupied space, turnover, and nonpayment of rent by tenants." This definition refers to market estimates of gross income adjusted for market vacancy and market collection losses. Your actual income may be very different from the "market."

When appealing your property tax assessment for an income producing property there are some expense items that you should not include in your income approach to value. The Appraisal Institute defines operating expenses as the periodic expenditures necessary to maintain the real property and continue the production of the effective gross income. Operating expenses can be fixed expenses that do not vary with occupancy, or variable, which generally vary with the level of occupancy or the extent of services provided. They include management charges, leasing commissions, utilities, heat and air conditioning, general payroll, cleaning, maintenance and repair of structure, decorating, grounds and parking area maintenance, security, supplies, rubbish removal and exterminating.  

There are non-operating expenses that you might be tempted to use in your income approach but the tax assessor will throw them out, or tell the board of equalization that you are including non-operating expenses. Some of these non-operating expenses are expenses that the IRS allows on your tax return, which causes some confusion.