We've all been to outdoor shopping malls with mulilevel buildings. Adding a second floor to a building doesn't change the buildings footprint, and is a great way for the builder to make considerably more money in rent rolls, than if they only had retail. If you are planning on purchasing or renting office over retail, chances are it won't be cheap. Having the luxury of popping out of your office and grabbing a latte at starbucks, is nice, but it will cost you.