Answer: If allowed to grow naturally, your Alexander magnolia can reach 25' in height and spread. This magnolia can be pruned and trained as a shrub, however, and you can keep it about half the the normal size with periodic pruning. The site you describe won't allow your magnolia to develop properly - you'll need to radically prune it to keep it from overtaking your house, and that much pruning is sure to reduce its ability to produce flowers. Three feet from any building is just too close. Try to find another site, 10-15' away from your house. This will provide you with an attractive shrub that will bloom reliably year after year.
Best wishes with your landscape!
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