Since you have already saved up the money, it would be best to use the cash to pay for the addition and avoid the home equity line of credit (HELOC). Taking out the HELOC would cost you interest, even if it is a small amount, and there is no reason to pay your bank the interest if you don't have to. The following two scenarios may help.
Scenario 1: Get the HELOC and Invest
Scenario 2: Pay Cash
Scenario 3: Get the HELOC and a mistake