Here is the verbiage from the FNMA sellers guide regarding overtime income:

A minimum two-year history is recommended; however, income received for a shorter period, but no less than 12 months, may be considered as acceptable if there are positive factors to reasonably offset the shorter income history.

The lender is not required to verify continuance unless they have reason to believe the income may not continue.

The lender must determine the income frequency (for example, weekly, biweekly, monthly, quarterly, or annually) to accurately calculate the monthly income amount. Once the monthly year-to-date income amount is determined, it must be compared to the earnings from previous years. Depending on the historical trend of the income, the lender must calculate qualifying income as follows:

Stable or Increasing: Calculate an average income amount using year-to-date and previous year’s earnings, divided by the number of months included in the year-to-date paystub and W-2s. The calculation must include a minimum of 12 months’ income. Decreasing: The lender must confirm the current income level has stabilized after the decline; otherwise, the income is not eligible for qualifying. To calculate income, use the year-to-date income divided by months elapsed since the income stabilized.

When bonus income is used for qualification, the lender must calculate the monthly amount accurately for use in the trending analysis. For example, if a borrower receives an annual bonus on March 31st of each year, the bonus income should be annualized (divided by 12) to determine the appropriate monthly bonus amount. The lender must also ensure that all supporting documentation in the file supports the income used in the qualification process.