Hi Daniel,
I was running into a similar issue when trying to finance my ADU in Walnut Creek. The options I discovered were;
1) Refinance my existing mortgage. Which wasn't an option because our primary mortgage is at 2.5% fixed. However, if you're willing to refinance, there are some ADU grant programs that will help cover up to $40k of the pre-construction cost (permits, utilities, etc.) for an ADU. Happy to connect and discuss further.
2) Second mortgage, I was pleasantly surprised by the rates offered by PNC for a second mortgage under 5.0% when we locked in, they call it a SHELOC. They had three options were we could pull out up to 80%, 85%, or all the way to 95% of the home appraised value. Discover also offers a second mortgage, there appraisal process was aggressively conservative.
I have also heard of various construction loan options but I didn't look to far into this option as the second mortgage worked well for us.