The rules for “grossing up” income are the same on FHA and conventional loans. You are able to use 125% of the income for qualifying purposes. Remember that the child support has to have a history of consistent payment and must continue for 36 months in order to be used in qualifying.
If you are a consumer, you should let the originator know the actual amount of child support you are receiving and not the grossed up amount. This will help ensure that the originator is using the proper amount when they run your loan through the automated underwriting engines.
As a general rule, you are able to gross up any non-taxable income.