"So in your opinion I should wait to depreciate the building and amortize the start up expenses the month they start renting."

You cannot depreciate something not yet existing and in use. Think of Depreciation as the loss of a little bit of value each year due to wear and tear. Your client is still investing in something yet to exist and be in use.

And since this is a Rental entity, without a Rental, you don't really have Operations yet, either.

"and have two assets called construction in progress and organization or start up expenses as assets."

Make sure they track Attorney costs related to the construction separately (as CIP) from those for the entity formation (as operations), as well.