How much value does a garage add to a house?
Unfortunately, it is difficult (perhaps impossible) to answer this question out of context. In order to get a sense of how a garage may improve your home's desirability, take a close look at your immediate neighborhood:
Are garages commonplace? Do they appear on most homes like yours, or would yours stand out?
Is parking at a premium?
Do you live in an area of the country that gets extreme weather?
Would adding the garage reduce open space or yard (that some buyers would find preferable to a garage)?
Check online at recent sales in your immediate area that have similar bedroom, bathroom counts, and square footage. Do garages appear to command higher sale prices?
Finally, you may want to consult a market analyzing Realtor and/or appraiser in your area to ask what his or her typical "adjustment" is for a garage on a home of your type.
In some areas of the country, a garage may not add much value to the home at all. . . in other areas, it may change the value by 30-50K or more. It all depends on context - and only a local professional familiar with your home and neighborhood can give you a better answer.