FHA loans require the mobile home be placed on a permanent foundation. A permanent foundation is defined as:

Permanent foundations must be constructed of durable materials; i.e. concrete, mortared masonry, or treated wood – and be site-built. It shall have attachment points to anchor and stabilize the manufactured home to transfer all loads, herein defined, to the underlying soil or rock. The permanent foundations shall be structurally developed in accordance with this document or be structurally designed by a licensed professional engineer for the following:
1. Vertical stability:
a. Rated anchorage capacity to prevent uplift and overturning due to wind or seismic forces, whichever controls. Screw-in soil anchors are not considered a permanent anchorage.
b. Footing size to prevent overloading the soil-bearing capacity and avoids soil settlement. Footing shall be reinforced concrete to be considered permanent.
c. Base of footing below maximum frost-penetration depth.
d. Encloses a basement of crawl space with a continuous wall (whether bearing or non-bearing) that separates the basement of crawl space from the backfill, and keeps out vermin and water.
2. Lateral stability. Rated anchorage capacity to prevent sliding due to wind or seismic forces, whichever controls, in the transverse and longitudinal directions.

Since most mobile home installations use a screw-in soil anchor the mobile home will not qualify for a FHA loan. The next area is the reinforced concrete footing, most installations use the a relatively thin pad which is installed on the surface or slightly into existing grade. These pads have no reinforcement and are not installed below the frostline, also the cmu piers should be grouted and not dry stacked. If you are considering refinancing with a FHA loan, you should do a little investigating to determine if you qualify. Use this link for additional information: HUD Manufactured Homes: Foundation Compliance