Lien position decides who gets paid first if a house ever gets sold out from under a borrower, and it's set by the order the loans were recorded. Your first mortgage sits in first position because it recorded first. Your line of credit sits in second because it recorded after. Pay that first mortgage off in a refinance and the line of credit automatically slides up into first position, which would put your new loan behind it.
No first mortgage lender will accept that. So before your refinance can close, the lender on your line of credit has to sign a subordination agreement. Subordination just means agreeing to stay behind the new loan instead of moving up. It's routine, it gets granted most of the time, and it's also the step that blows up refinance timelines, because almost nobody starts it early enough.
Start it the week you lock your rate. The request goes to your lender's subordination department rather than to your local branch. Turnaround commonly runs two to four weeks. Some are faster. A few are a lot slower, and there's nothing you can do about that from the outside except get in line sooner. Ask your loan officer for the subordination package on day one, and fill it out that day.
The lender is going to look at you again before agreeing. They check the combined loan-to-value, which means every loan against the house added together and divided by what the house is worth. They check your credit, and sometimes your income. Many order a new valuation, often a computer estimate rather than a full appraisal. There's usually a fee, and lenders quote it up front, so ask what it is when you request the package.
They can say no. If the combined loan-to-value is too high with the new loan in place, or your credit dropped, or the value came in soft, you get a decline. Then you have three choices. Pay off and close the line of credit as part of the refinance, which stops the strategy until you open a new one. Roll the balance into the new first mortgage, which turns a flexible balance into a fixed payment you can't redraw. Or keep the mortgage you already have and skip the refinance.
They can also say yes with conditions. The most common one is a smaller credit limit. They'll subordinate, but your $150,000 line of credit comes back at $100,000. That's their risk decision and it's rarely negotiable. You can still ask what number they would approve before you accept the first one they hand you.
Two habits make this go smoothly. Keep the balance you've drawn low in the months before a refinance. It helps the debt side of your file, even though most lenders measure combined loan-to-value against your full credit limit rather than your balance. And tell your loan officer about the line of credit in the first conversation. The refinance applications that miss the rate lock are the ones where a line of credit turns up in week three. The rest of the timing problem is in refinancing in the middle of a chunking plan and applying for a mortgage while your line of credit is open.