home addition mortgage? Blended?

Please excuse me if this is the incorrect forum to post this question.
We would like to have an addition built on our house. I know about HELOC's and Home equity loans. I was recently talking to an acquaintance who mentioned a "blended mortgage". Here's how he described it. Talk to your mortgage company. See if they will do a construction loan for the addition. Pay the going rate for the construction loan. When it comes time to close the construction loan and make it permanent, have them roll it into a 30 year mortgage and then "blend" my previous mortgage (at 5.75%) with the current going rate. I'm not sure if I can do this with another company other than who holds my current mortgage and get some of the benefit of my current mortgage rate.
Anyone know if this can be done? I will certainly call my current mortgage holder (Chase), but want to make sure I have the terminology right and can explain it correctly.
TIA,
B
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HELOC's
time
into
the
mortgage
holding your mortgage, could you? All you can do is ask, and see what your current lender says. Depending on what the current interest rate is, they may not have a financial incentive to do this, but it never hurts to ask.
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And the other problem is that Chase needs to be still holding the mortgage. Many of these get packaged and sold, with the mortgage company only doing the servicing. But it can;t hurt to try, certainly Chase should have some alternatives to suggest.
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