Below is what the guidelines say for FHA:
Conversion of current primary home to investment property when buying new owner occupied home.
*Both the current and the proposed mortgage payments must be used to qualify the borrower for the new transaction
*Rental income on the property being vacated, reduced by the vacancy factor, may be considered in the underwriting analysis under either of the following circumstances:
1/ Relocations - When the homebuyer is relocating with a new employer or being transferred by the current employer to an area not within reasonable commuting distance.
2/ Sufficient Equity in Vacated Property - The homebuyer has a loan-to-value ratio of 75% or less as determined by either:
***A current (within 6 months) residential appraisal
***Calculating the LTV by taking the current unpaid principal balance of the existing lien to the original sales price of the property.

I hope this helps. Please call if any questions.

Wayne Laverdiere
Envoy Mortgage
Dover, NH
(603)749-5626