ADUs in North County San Deigo

[Michelle Park Senyard]

In addition to our long-distance real estate investment, my husband and I want to build out an attached ADU out of our 3rd garage port. Does anyone have experience, successes, or failures they learned from, while building attached ADUs on their property, that they can share? Other questions we have for people with experience in ADU building:

• Are there banks people recommend for getting a reasonable HELOC interest rate?
• We've had a few ADU-building companies come to look at our potential ADU lot size and they've quoted us $270K-$400K which is outrageous, so our hope is that we can bring the total cost down to $100-$200K max if we do the interior design and project managing ourselves. Any tips on how to cut down costs without going the all-inclusive ADU builder route?
• How are comps for homes with ADUs? How much value have they added to your property? 
• Has it been easy to rent an ADU out in North County San Diego?
• Anything we should be aware of?

Thanks in advance!