I understand you pro-rate expenses when renting out a property with Airbnb, my question is do you include all the time it is available to be rented, even if no one is at the property, like during an off-season? The property is advertised as available, and accessible for the u year. Thanks.
You can deduct your expenses if the room was available for rent (so you don't have to have it rented, but it must be available for rent... for example, you were already advertising it). It's ok that you have no income. You can deduct the expenses, even if no rental income is reported, as long as the room was available for rent.
If, however, the room was not available for rent... you cannot deduct the expenses. All your expenses (except property taxes and mortgage interest if you have one) paid BEFORE the property was listed as available for rent are simply added to the cost of your rental property (in your records). The total amount is called adjusted basis (not just the cost, but the cost plus all your pre-rental expenses).
Once the property is available for rent, you will start depreciating the rental and use the "adjusted basis" - the amount which includes your pre-rental expenses.