Conforming mortgage guidelines are the Home Loan Rule Book, delineating between applicants that approved for a mortgage and those that do not.

Effective today, the rule book just got a little bit tougher.

According to Fannie Mae, homeowners converting their primary residence into a second home or investment property will be subject to additional underwriting scrutiny. Fannie Mae is leery of lending to people that may be over-extended.

The complete underwriting update is available at the Fannie Mae Web site but some of the more important points are summarized below, divided into Second Home and Investment Property.

Second Home Guideline Changes

• Without 30 percent equity in the second home, mortgage applicants must have 6 months worth of PITI reserves for both properties in their bank accounts.
• With 30 percent equity, the PITI reserve can be reduced to 2 months.

Previously, there was no minimum reserve requirement. Now, a second home buyer needs to know that they can carry that property for a period of time with their current savings.

Investment Property Guideline Changes

• With 30 percent equity in an investment property, 75% of the monthly rental income can be applied toward the applicant's monthly household income.
• Without 30 percent equity, rental income may not be applied to the applicant's monthly household income and 6 months PITI is required for both properties.

Previously, 75% of the rental income was allowable regardless of equity, and minimum reserve requirements were 2 months. Due to the number of "investors" who bought property without equity, and then walked away from the properties when they were unable to rent them, some method of involving the investor in the success and failure of the investment was sought.

Even though just a small percentage of Americans own second homes or investment properties, the conforming mortgage guideline changes impacts homeowners everywhere.

This is because more restrictive guidelines lead to two separate, but concurrent, outcomes:

1. The demand for homes reduces because fewer buyers qualify for mortgages
2. The supply of homes increases because fewer sellers can refinance into more affordable home loan

Less demand and more supply places downward pressure on home prices.

Now, remember that mortgage guidelines continuously evolve and what's accurate as August 1, 2008, may not be accurate six months down the road. In other words, confirm what you're reading about mortgages online with your loan officer before making any real estate-related decisions.