For a passive rental activity reported on SCH E, start up costs are not allowed. So any expenses incurred in "preparing the property for rent" for the very first renter, are not deductible at all. Now do not confuse this with property improvements. That has nothing to do with start up costs at all.
For an active short term rental activity that qualifies as a trade or business, which gets reported on SCH C, start up costs are claimed as such, in the first year you are "open for business". It does not matter in what year those startup costs were incurred either. But again, do not confuse start up costs with property improvements.
Since this is a short term rental, and assuming it qualifies as a trade or business, I assume it will be furnished. I also expect you to depreciate the furnishings beginning on the date they are placed "in service". If the furnishings were purchased new *and* the tenant was the first to use them, then you'd have a choice to expense the furnishings if the total cost of those furnishings is less than $5000 per invoice that the purchase of those furnishings are listed on, or to depreciate the cost over 5 years.
However, (and I'm not 100% sure on this) since the furnishings will be "used" (by you) prior to being placed in service, I do believe you can't expense them and have to depreciate them over 5 years, based on the FMV of that furniture on the date it's placed in service. If I'm wrong on that (and I may be) I'm sure someone will jump in here.
