Your house appraises for more today than it did when the line of credit was written, and the credit limit hasn't moved. A home equity line of credit is sized at closing and stays that size until somebody asks for a change. Lenders don't hand out increases on their own.
The limit came out of a combined loan-to-value calculation. That means the lender adds the mortgage balance and the line of credit limit together, divides by the appraised value, and holds that result to a ceiling, usually somewhere between 80% and 90%. A $500,000 house with a $300,000 mortgage at an 85% ceiling supports a $125,000 line of credit. If that house appraises at $600,000 two years later and the mortgage is down to $280,000, the same 85% supports $230,000.
There are two ways to ask and only one of them you want. A credit line increase on the existing account keeps your rate, your margin, and your draw period, and changes nothing but the limit. A refinance of the line of credit replaces the account, which restarts the draw period and re-prices the margin at today's terms, and today's margin may be worse than the one you already have. Ask for the increase by name, and ask what happens to your margin if they can't do it that way.
What they look at is the same list as the first time, which I walked through in a video on getting a line of credit approved. Credit score and recent inquiries. Income and debt-to-income. Payment history on the existing line of credit. And the property value, which they'll get from an automated valuation model, a drive-by appraisal, or a full interior appraisal, in rising order of cost and accuracy. An automated model is free to you and often comes in low, because it has no idea you redid the kitchen. If the number comes back short, ask what a full appraisal costs, usually somewhere from $400 to $700, and whether you're allowed to order one.
Time it. Don't ask inside the same 90 days you're applying for a mortgage, because the credit pull and the new limit both land in the other lender's file, and applying for a mortgage while your line of credit is open covers what that does to the approval. Don't ask while the drawn balance is at its peak either. Pay it down first, wait for one statement to show the lower number, then apply. A line of credit sitting at 90% drawn reads like somebody who needs money. One sitting at 10% reads like somebody managing credit.
Before you take the increase, decide what the extra capacity is for. A bigger limit doesn't shrink the payments and it doesn't make the chunks any safer. What it buys is more room between your working balance and the ceiling, and that room is what keeps you out of trouble when a rate moves or a furnace dies. Never max your line of credit is the rule the extra room exists to protect.
Call the lender's home equity department rather than the branch, and use the phrase "credit line increase on my existing HELOC." Ask what valuation method they'll use, what it costs, how long it takes, and whether any of the existing terms change. Get all four answers before you authorize the credit pull.