We are also looking into this and it seems that the options are: HELOC, cash out refinance or construction loan (which then either needs to be paid back or refinanced into the existing loan balance - some banks do construction to permanent loans with only one closing, and those tend to be 5 or 7 year ARMS). With construction loans, there are costs for bank inpsections at various points, so I would think that the HELOC or a cash out refinance make the most sense in your situation.