Thanks for all the great replies, suggestions, and additional information.
We've already made the purchase. Whether the garage is attached or detached is not a make-or-break deal. I'm just trying to understand my options and do what is best and most cost-effective, given that I will have to work within a budget.
1) Our house has not yet been built. The pictures are of the same model that we'll be building. The neighborhood is all minimum 1-acre lots. My original plan was to build a detached garage. Then discussions with family/friends led to "why don't you just add on to the existing garage?" That'll mean one less wall to build, access directly from the house, already having ducting for hvac, less driveway concrete, etc." That all sounded good and this is why I'm inquiring as to what is involved. From the responses I'm seeing here, it's sounding like an attached garage is likely to be cost prohibitive.
2) The builder won't do it. I've already asked the builder if they would build a detached garage for me and they said no. Whether attached or detached, it would be so much more convenient and cost effective if they would build it. They're not a "custom" builder. Ironically, they offer an option to build a Casita (stand-alone in-law dwelling) on your property. I also asked if they would build the Casita structure and allow me to do the inside (no kitchen, no bedroom, etc.) Again, they said no.
3) I've been dealing with HOAs for decades; a blessing and a curse, a necessary evil. It seems in this case, the builder will be the sticking point, not the HOA.
4) There are a number of detached garage/workshops already built in the neighborhood. So, I understand the general issues and restrictions - no metal buildings, same materials/aesthetics as the house, etc. I don't anticipate any problems here because I really like the buildings I've seen. See 1st 3 pics below - this shop looks great but may be a little bigger than my budget will allow.
5) Builder in the hood. There is a guy in the neighborhood that owns a construction company. He's already built his own garage that is half garage and half 2-bedroom apt. His place is the last picture below (with the fence in front). Given that he lives in the neighborhood, is already familiar with the HOA, and has already built his own shop, I may hire him for this build.
5) Construction Manager. We're meeting with the Construction Manager on Friday (4/17) to go over the build. Asking about an attached garage will be my first question to him. Are there other issues/questions I should ask him that have not been mentioned yet?
Thanks for all the help and thanks in advance for all the future help with this!