Have you ever seen a fence with a post about an inch from a building wall? That is great to reduce decay, but it also makes it a separate structure and depreciable as a fence instead of an extension of the building. Or so they say. I find it hard to imagine it would actually matter to an auditor. And yes, best source is the IRS. Appliances for example have their own depreciation schedule, etc. What I do is on any larger purchase just quick search in the depreciation tables to help make the decision on how/whether to do it. But many items are now 100% depreciable first year with the new tax codes, and I am not up to snuff on that at all.