Because if you know from the get-go that you are not moving into that property, and you purchase it with a home loan that requires owner occupancy, you may be committing fraud. If the lender knows what you're doing, he or she could be considered to have conspired to commit fraud as well. Once again, it's not illegal to invest in real estate. What is illegal is signing a note in which you say you intend to move into that home when you have no intent to do so whatsoever. Ask them to help you figure out first if you can build anything on top of your garage, what it is that you are allowed to build, and if you can lease it out.
Many communities have strict prohibitions on building rental apartments onto single family homes.
There are several issues you'll need to address. Most communities will only let you build a certain amount of square feet on a particular size lot, and this changes from city to city. For example, on a lot that is 50 feet by 125 feet, you might be able to build 3,500 square feet in one area, or 4,500 in another. You'll have to figure out what the ratio is for your community and if you meet that standard with what you're building. You'll also have to look at the local lot setback and building requirements.