This is the most current guidelines for FHA with regard to ADU;
Most recent guide for the captioned situation:
An accessory dwelling unit (ADU) is defined as a habitable living unit added to, created within, or "detached" from a primary single family dwelling and contained on one lot. ADU�s are commonly understood to be a separate additional living unit, including kitchen, sleeping, and bathroom facilities. ADU�s are subordinate in size, location and appearance to the primary home
and may or may not have separate means of ingress or egress. An attached unit contained within a single family home, also known as a "mother-in-law apartment," or a �garage apartment� that "may or may not" be attached to the primary residence are the most common types of accessory dwelling unit. An accessory dwelling unit sometimes involves the renovation of a garage, basement or a small addition to a primary residence.
The determination of whether or not an ADU is a second dwelling unit is to be made by the appraiser and indicated in the site analysis section of the report where zoning, highest and best use, and legal use are addressed.
The fact that an ADU is rented or generates income should not categorically result in a determination that the property contains two dwelling units.
An accessory dwelling unit may or may not have separate electrical service. The appraiser will make the determination of whether or not an ADU is a second dwelling unit utilizing the criteria stated above in question number one.
FHA does not take the position that an appraiser should conclude the presence of two electric meters necessarily constitute a two dwelling unit structure and not an ADU.
This was sent to me from FHA resource center on Monday 10/04/2010. I was working on a similar project of a SFR with a detached ADU on separate meters, well and septic and was concerned about the utilities. I called the FHA resource center in Philadelphia as well as my home office in CT and received this same answer from both offices. It really comes down to whether or not it is legally a SFR or 2 unit, HBU. In my situation since the site does not meet current minimum zoning requirements and set backs the property can not be subdivided to 2 SFR and there is no multi-family zoning permitted in the town. So it is a SFR with ADU however it does not meet FHA minimum requirements due to location of well and septic for both units.