Low drive-by appraisal for primary residence when applying for Heloc

[Kantcho Traykov]

Hello, everyone. Looking for some advice on what to do in this situation. My wife and I bought our primary residence in 2021, which was a new construction at the time. So our house is listed everywhere ( public records and Zillow/realtor.com) at 1615 sq ft, 3 bedroom 2 bathroom house. However....., even as a brand new construction our house has had a complete bonus room that is cooled and heated above our garage and there is stairs that were built leading to the bonus room from the first floor of the house (inside). So for whatever reason, the sq footage of our bonus room isn't showing anywhere in any documentation of the house which is about 300 sq ft missing from our house so our house would be 1915 sq ft. So, long story short, recently we have been shopping around for a Heloc because we have built some equity in the house and the market has grown in the area and I am looking to take action into real estate investing. So when we applied for helocs and stuff and we had those drive by appraisals and stuff, we keep getting a lower appraisal than I could ever even imagine. And so I feel like the extra living space, our bonus room not showing any of the documentation is hurting our appraisals. On top of all of this the comps that they show in the report in our surrounding areas are nowhere equivalent to our house. They show houses that are 20+ years older and some of them don't even have garages etc... that I feel are unfair comps for our property. Our house is less than 3 years old. So basically, my question is how should my wife and I proceed to try to get a fair appraisal and how do we go about having the 300 sq ft added to our house that's missing from any documentation? Thank you in advance for the answers and I apologize for rambling.