If your property has enough room, you may discover that having an extra parking place inside for another car or room for a little workshop requires adding onto the garage. This could be the easiest project when done correctly. First you need to prep the area where the add stall we be and then make the fittings for pouring the cement flooring.
Once you have the fittings poured it is time to construct the frame of the garage, do this with two by fours and plywood. Adding the siding afterwards when the inside is completed or before, it is entirely up to you. You do need to do some added work to attach the new section to the existing garage in order to rip the wall down, unless you want the wall to remain up and just add a doorway to the existing garage.
One major concern when building the garage, the size and the room you have allotted for an extra garage stall. If the room to the side does not meet city building codes, you may decide to extend the length of the garage from an existing stall. This works nice if you just wan to small workshop or an area to work on the old car you brought and want to restore.
Therefore, adding on to the garage can be done to the side or towards the back into your backyard. Adding on to the garage works with the existing building when you plan and design the new addition down to the material needed.