I absolutely agree with the majority of the posters. No one can say with any certainty what the value of an additional garage is unless there is specific paired sales data available.
The question I ask most homeowners is "what are YOU willing to pay for the difference?" I had a case where a seller complained about the appraised value, stating that I was "$50,000 low". The only difference between their home and one of my comps (within blocks) was their yard was 50' deeper (same width/road frontage) and was partially fenced. I asked how much the fence cost ($8,000) and then asked them how much they used the last 50' of their yard. All of their amenities were near to the house, like the patio, shed and swing set. The owner reluctantly admitted that the most distant part of the yard was not used regularly, but did provide extra privacy. I then asked if they (honestly) would think the average area buyer would pay a premium of $42,000 for privacy, and if they would list the property as such if they decided to sell.
That ended the conversation.
You might just have to ask a few realtors---if paired sales are not available---just what they think an extra garage contributes to the value.
And as most posters agree, location is a factor. In an urban city where parking is impossible, a garage may be worth $50,000. In a perpetually cold climate, a garage may be worth $20,000. In Florida or California, a garage may be worth $5,000. In a state where everyone parks their car on the lawn, a garage may be worth nothing.
