I read a post on accurate comping. The author showed me how to make relative ARV comps(comparables) closer to the property I'm analyzing.

He’d take three of the closest comps then find where the comps differ and increase or decrease their value based on the prop he’s analyzing.

Ex. He would calculate the cost of an extra bathroom($1000 per additional bathroom) or additional sq footage into his comp to match the features to the home hes analyzing. For a more accurate comp not just a ballpark estimate.

My question is how did he know how much an extra room would cost or the additional sq footage would equate to? I have a feeling I’d only know these answers if I was an experienced investor. Is there a way for a new investor to calculate these factors into my comps for a more accurate comp analysis. Is there a database or guide that gives a price to each of these variables?

P.S - He goes into deeper detail with home age and garages also being factored in but I feel like if I could Atleast learn the most important stuff like rooms and square footage I'd have a better idea of what my ARV would be. And feel more confident to act without fear.

Please help and thank you for any advice!