Real estate appraisers are about to face new rules that could change their business, BusinessWeek reported.

On Jan. 9, Fannie Mae and Freddie Mac announced revisions to their Home Valuation Code of Conduct. The changes will affect lenders who want to sell their loans to the two companies, which means they will apply to a huge share of the U.S. mortgage business.

As of May 1, lenders – rather than mortgage brokers and realtors – must select appraisers and are not allowed to influence appraisers by withholding payments or promising future work. If lenders have in-house appraisers, the bank’s loan-origination department is not allowed to influence appraisers’ valuation decisions or supervise their work.

These changes follow accusations that appraisers regularly approved a doubling or tripling of home values, which contributed to the housing boom and then bust. Fannie Mae and Freddie Mac’s revisions stem from an agreement reached last year among the two mortgage giants, the New York State Attorney General’s Office and the Office of Federal Housing Enterprise Oversight.

In a conference call organized by the American Appraisal Institute trade group, Gerhard Morell, an appraiser with Northern Colorado Valuations, said many independent appraisers depend on referrals from mortgage brokers for work. Fannie Mae and Freddie Mac’s changes could cause more appraisers to turn to one of about 200 appraisal management companies, which handle appraisals on behalf of lenders but often pay appraisers less money than they would make on their own.