My wife grew up in a neighborhood where their ranchers and townhouses had attached garages. Many folks converted their garages into additional living space. In her particular neighborhood, doing so generated a 'realized loss' in the value of the home. The homes are all fairly identical with the exception of pools, additions, and 'end townhomes', and such. All of the differences can be 'calculated', and even w/those homes, a finished garage generated a lower selling price.
Not that I'm trying to discourage you but you might want to ask a Realtor or 3, or even a recent homeowner that bought a home (like yours w/a finished garage) to see how it affected the price. It could be either a +, a -, or a wash. WHO knows w/o asking. In any event, I thought I'd mention it. Of course, it's cheaper than building an addition but an addition has a MUCH better chance of adding value.