How long do we need to wait before modifying or adding on to the new facility?
If this is a true new unknown requirement there is not a time limit. This new requirement should not have been known at the orginal time of construction. The new need should be a new unit, new mission, etc.
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Looking at this it sounds like your are trying to increment this construction; that is illegal. You would have to wait at least 5 years before adding to the facility. If you don't think this will pass an Inpsector General inspection, don't do it. This should have been programed under the MILCON.