I'm not sure I understand how it doubles your cash flow to build a garage? Can you expand on this?
I'm assuming you mean convert the garage into a secondary unit? Also what is becoming of your current primary residence, if you own it; it could be turned into a rental. If you are considering converting the garage it will depend on your local market, generally I don't think it is a good idea because people want a place to park their cars out of the weather. Also it will depend on what type of property you are thinking of purchasing, if it costs the same as a garage conversion, it may not be in the best of areas.