The forecast on value increase can only be a guess. It's all going to come down to how well the design complements the property and the local neighborhood, also it will be highly dependant on the potential buyer. When we sold our property in TN 5 years ago it had a very nice 30 x 40 with three overhead doors that had a stone wainscoting and metal colors to match the house. Our realtor actually targeted car enthusiasts in their marketing which I'm sure helped find the right buyer.
"Democracy is two wolves and a lamb deciding what to have for dinner.
Liberty is a well armed lamb contesting the decision." Ben Franklin