It's easy to get a permit that is good for 1 year. Yours will be inexpensive as they're normally based on square footage. Just tell them what you are up to and they will want a plan drawn up they can look at and then they can give you the OK to proceed. This is easy to do though, with most information available on your cities website, or you can pick up the information at the cities building inspection department. In my town, you can actually do it on a piece of 8.5x11 paper with a pen or pencil type drawing. They will ask you to draw it to scale though, however, if it isn't, they'll accept it anyway. Just make sure any dimensions you show on your drawing are accurate. For one thing this is how they will know how much to charge you for the permit. The good thing is, you know from the get go, that you're doing it right and don't have to worry about re-doing anything, as you already have the cities blessing with the plan they signed off on.
I am especially concerned about the water.
You have to obtain a permit for water prior to hooking up to the city water supply. This is very important, especially since 911, as municipalities are under strict observation by the EPA now. Municipal water supplies are considered a target for terrorism. It's a simple matter to contaminate the system in your neigborhood by inadvertantly creating a cross-connnection or by using an improperly installed vacuum break/backflow preventer. Due to the height of your structure, and the involvement of your neighbors with the city already, they will want to keep a close eye on the installation I suspect.
This is the way it is in Colorado anyway. Sounds like a cool Tree house! I hope you put in an Internet connection while your at it.