A couple of thoughts on this.
I would expect to keep a record of the amount depreciated with the room used as an office, then you would start depreciating the house as appropriate when it transitions to a rental. When you sold the house, you would need to figure the total amount of depreciation taken between both types [ office and rental] and that # would be added to the profit made on the house, for figuring if / what taxes, are due on a long-term appreciated asset that gets sold.
Consider if it would be worth the hassle, to keep the records, prove exclusive office usage in an audit, etc . I think many here probably do depreciate value on their tax returns in regards to an extra room / home office.. We opted Not to. Wife does deduct 1/2 the internet bill , office supplies, and mileage, in her profit and loss statement. But on the home office - she is sometimes working at home less than 40 hours, as she spends 2 days a week at her other job at the employers commercial business location , and her own business has ebbs and flows depending on quarterly reports,etc.
She does not see clients in her home 'office", and she felt if claimed as an office, than she shouldn't be using it's closet to store her personal items that weren't business related.
Also for us, the room she uses, is only about 1/10th [or less] of the total square footage in the house - so that means we'd be depreciating only <10% of the mortgage principle and interest payment, maybe less than 10% of the insurance portion, since we have an garage + deck, which don't count as livable square footage, but are still covered under a standard homeowners replacement policy cost. Real estate tax i imagine would not be depreciable/ deductable.
If and when we sell the house, whatever proceeds we get wont be complicated, or reduced by depreciation claimed over the years.
I'm sure it is a great tax reduction strategy for some, for the reasons listed though we decided, for us, it wasn't worth the hassle.
