You need to apportion the property tax and depreciation. Generally if your house is 5000 square feet and 1000 square feet is used exclusively for your business, you could reasonably deduct 20% of your property taxes and 20% of your depreciation.
For depreciation, you also need to keep track from year to year since you may eventually sell your home at a profit. Depreciation on a home does not include the land value, but it does include all the expenses of acquiring your home. However, if you built the home office as a separate project, you might just take the entire cost of the build and depreciate it as a business expense, since it is separate. And obviously then the cost of this build would not include the cost of the land, which does not depreciate. Depreciation is typically straight line for 27.5 years so you would take the basis (original cost), divide by 27.5, and deduct that amount each year. If you are using a % of the whole, you then apply that %. So a 275,000 build (not including land) would be $10,000 a year depreciation. If that were for your whole construction and the business has sole use of 20%, you would reduce that to $2000, etc. This is covered in the IRS publications on depreciation.
