As long as the HELOC was taken out within 90 days of completion of construction, you can consider both the HELOC and the mortgage as fully deductible acquisition debt.

In Turbotax, enter both loans separately, you will be asked for each loan, if it was used to buy, build or substantially improve your home.  Say yes.  (Unless part of the HELOC was used for other things, then it is only partly deductible and you will have to do a calculation.)

The answer provided did not answer the question.  There were 3 loans: (1) the original construction loan, (2) the unsecured loan that I needed to get to pay for the over-budget costs, and (3) the HELOC.  I could not get the HELOC until the original construction loan transformed into a regular mortgage.  The bank would not transform the construction loan into a regular mortgage until the builder agreed that he had been paid all his costs.  I needed to get the unsecured loan so that I could pay the builder all his costs.  I received 1098s for the original construction loan (which converted to a regular mortgage) and the HELOC (which paid off the unsecured loan).  But I did not get a 1098 for the interest on the unsecured loan.  Is that interest deductible?  The monies for the unsecured loan were definitely used for the building costs. Thanks!

That's unclear, but probably not.  To be deductible interest, the loan must be secured by the property.  In the case of a construction loan, where there is no house, I assume that the loan is still secured by a lien on the property itself and recorded in the county records office.  That makes the construction loan deductible as a mortgage.  A completely unsecured loan would not be deductible even if the expenses were used to build your home.

[Now, if construction loans are also completely unsecured, that would seem to be a case where IRS policy goes against the written law--where policy was interpreted in a way to be beneficial to home builders in what might be a common-sense way but that is not supported in the text of the actual law (which says the loan must be secured by the property).  In that case, your unsecured loan might also qualify, it would be a matter for interpretation by an auditor or the tax court, if you were audited.]

but the interest related to the unsecured loan is NOT tax deductible.,