The risk of a security's expected maturity lengthening in duration due to the deceleration of prepayments. Extension risk is mainly the result of rising interest rates, and is generally associated with mortgage-related securities. The opposite of extension risk is prepayment risk, which generally occurs in a declining interest rate environment, and is associated with people paying off their loans too quickly.
As interest rates rise, the likelihood of prepayment decreases as people will be less likely to refinance their homes. If the loans in a pool underlying a mortgage-related security are being prepaid at a slower rate, investors are unable to capitalize on higher interest rates because their investments are locked in at a lower rate for a longer period of time. As interest rates decline, however, the likelihood of prepayment increases because refinancing becomes more attractive. When a loan is refinanced, the original loan gets paid off, and investors then have to invest their proceeds at the new, lower market rate.