If you intend to use the Container House for your primary outdoor storage, you will need to add at least 3' to the back. Some of this is for the actual storage, and some to enable you to walk around the parked cars. Bigger is better. We generally recommend five feet - two feet of storage and three feet to walk around to get to the storage.
Building a garage is a major undertaking. It is an addition to your house and should be taken as seriously as any other addition. A well-designed and constructed garage can add considerable value to your property as well as useful functionality while you still own the house. A poorly designed garage can easily detract from your property, lessening rather than increasing its value.
Building a garage is generally not something to be done without serious professional planning. You will need detailed plans for the project. Oh, we know that 50 years ago good old35 Uncle Gus slapped up a garage on the Ol' Homeplace over the weekend without plan one, and it's still standing — more or less. Times have changed, though. All garages need building permits in most communities. This includes a site plan to ensure that the garage does not intrude into zoning setbacks and does not overbuild the lot. There are restrictions on how much of your lot you can use for buildings, and this is determined by your zoning and the size of your lot.
Good plans are not only necessary to get building code approval, but ensure that the garage is structurally sound, contains the features you need, and is designed to complement your house. Just as important, it helps ensure that you get only the features you need and that you are not paying for facilities you will never use.
Most existing detached garages cannot be effectively heated because they were not built to be heated — there is no insulation under the slab and no thermal break separating the slab in the heated interior from the footings exposed to frigid exterior. You can, of course insulate the walls and ceiling, and this will help some. But, adding insulation to an existing non-insulated slab is virtually impossible to do cost effectively. Insulated and non-insulated slabs are built quite differently. If you intend to heat the garage, or may add heat to the garage later, an insulated slab is a must. Without floor insulation, cold simply flows in through the garage floor. So, if in doubt, insulate the slab. (For much more on insulation, see Insulating Your Old House.)
Plan on adding electricity if you want lights, outlets or an automatic garage door opener. For detached garages, adding electricity is getting fairly costly since recent code changes mandate new safety features including a shutoff in the garage and dual masts for overhead lines - one at the Granny House and one at the garage - or underground wiring. Neither of these options is trivial and can add substantially to the cost of a basic garage.