Unless you are a Real Estate Professional reporting your rental property as a Self-Employed Business, on Schedule C, you don't need a business code, but if you are, you might choose 531100, for Lessors of Real Estate and Dwellings.
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Yes, you can use 999999 for the business code. Since this is not an ongoing business for you, code 999999 will be good. This will not cause any red flags to be raised with the IRS.