FHA mortgages are home loans that allow for a lower minimum credit score and down payment than many conventional loans. These loans are actually funded by a mortgage lender, but backed by the federal government. In order to purchase a home with this type of loan, the property must go through the FHA appraisal process.
In order to back a mortgage, the government needs to make sure the loan is a sound investment, which is why they require a special FHA-specific appraisal. This appraisal serves two purposes: The first is to assess the market value of the house. The government will want to ensure the loan amount they will be backing is equal to or less than the market value of the home. The second is that they will also want to assess the home's condition, longevity and livability. It's this dual purpose that distinguishes FHA appraisals from conventional-loan appraisals.
FHA appraisers consider value, but they must also confirm that the home conforms to its minimum property requirements, which include safety and other issues: the absence of lead paint, properly functioning appliances, etc. This leads them to places a regular appraisal may not go: Attic or crawlspace inspections are required for FHA appraisals, for example, but not necessarily for conventional ones.