So all great questions, etc. We've already worked thru most of them and at the appropriate time (aka, when I have time) I can share details of why this makes sense financially. We've worked thru the setback items too, or at least we know how much room we have to work with.
MTown - your thought is perfect on house vs garage size, our main level is just shy of 2000 sq ft and the basement is the same, with about 1600 sq ft of it finished. So, we're a rather large house to only have a 2 stall garage - hence the biggest reason to do this.
Basically, we finally spoke to someone that doesn't have their head in their *** - and provided some excellent metrics. For the last 2 years, there have been zero houses of our size that have sold in our area, with just a 2 stall garage. None. But there are plenty of our size that have sold with a 3rd stall, easily for what we are asking and more.
Do we know for sure we are adding it? No. But we must look into it as it looks to be a smarter decision than a dramatic price drop.
So we can discuss the do it vs don't do it later, just need some good references at this time.