You probably want to determine what the supply/demand balance is at your local fields in the immediate future. If there is an oversupply, you may be able to defer the issue until the airplane is ready for final assembly. Alternately, you may have needed to anticipate your hangar need 5 years ago for your name to make the top of the waiting list in time for your airplane.
If there are waiting lists at your local fields, find out the rules and get your name on there. Also, check back routinely, because some airports forget about you (despite your name being on a list) if you don't keep in touch.
If there are lots of empty hangars in your area, or if the FBO's tell you something to the effect that hangar availibility "isn't a problem", great. Just make sure you follow up every 6 months or so in case things change.
I know that here on the NW side of Atlanta, people are driving 50+ miles to find hangar space, which really puts a damper on their flying. If you can avoid that type situation by taking action now, you'll be a much happier aircraft owner a few years down the road.