Who said you can't do it?
If it is in the guidance you received from the water company, then you're better off going to a friends place and doing it all there. If it was just some random person in your neighborhood saying you can't wash your car there, do some research and find out what the exact restrictions are. Honestly, some of these "community leaders" do things that are way out of their scope of responsibility. If I am washing my car, a cop has to tell me I am doing something wrong before I will listen, not some random neighbor. We live in America, we have freedoms. I know I have read people that say they are approached in their neighborhoods by haters that don't like seeing people with a nice car and they tell them not to wash it there. Also some environmental nuts say the same thing because of the dangers and damage to the environment. Whatever... I know in my town I cannot water my lawn or wash the car on certain days of the week (Thursday and Friday I think), but that comes straight from my water company.
The problem with washing it and then driving it somewhere else to clay/wax you'll have a problem with dirt/bugs attaching to the car again. Plus, when you remove the wax at the carport or friends place, the car will be susceptible to more damage since there is no wax/sealant protecting the clearcoat and paint. It's better to do it all at one place and get it done.