Technically, if you so much as replace your water heater you're supposed to get permits. In some states, if it costs more than $100 in materials you need a permit (that law is about 40 years old or more)
In reality, a lot of people just do it - it depends on your gambler instinct (if the Building Permit people catch you, they can "red tag" the construction, and throw you in jail if you persist.)
Part of the decision, assuming you're an outlaw at heart, depends on how brazen you are, how good a terms you are on with your neighbors ("hello, building dept.? Did you know that Mr. Rushfan33, at 33 Rushfan Lane, is obviously re-modeling his house without a permit? Bastard laughed at my new car last week too, but that's another story...)
If you do anything to the OUTSIDE of your garage, like get rid of the doors and wall it up, it will be obvious with just a drive-by, which some cities have people do from time to time.
Generally though, if you're sort of subtle about it and keep on good terms with your neighbors (like by telling them you're trying to keep things quieter for THEM) it will go better.
Those were just a few things off the top, hope it helps you decide... Steve
