If I start a business in the middle of the year, when deducting annual expenses, can I deduct the full annual expense or only a prorated portion after business start?

[davidwmccandless]

In my specific case, we have a cottage on our property that we started Airbnbing in May. We have annual expenses like flood insurance. I understand a portion of the flood insurance can be deducted e.g. 10% if 10% of our square footage is the cottage and we only use the cottage for Airbnb guests, not personal use. But since May - December is only 7/12 of the year, would I only be able to deduct 10% * 7/12, or a full 10%?

[Critter-3]

Prorate.