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If your rental activity qualifies as a business activity, you don't need to make the safe harbor election, see #3 below.  

Note that for rental activities not conducted by a real estate professional or not rented to a related party, a rental activity can fall into three categories for qualified business income deduction QBID:

1. A rental activity not eligible for the QBID because it does not meet the safe harbor criteria and is otherwise not considered a trade or business;
2. A rental activity eligible for the QBID because it does meet the safe harbor criteria (Safe Harbor statement required); and,
3. A rental activity eligible for the QBID because it is considered a trade or business by the owner (Safe Harbor statement not required).   In this case, you meet the real estate professional definition and work more than 750 hours in your business each year.   Please this executive summary regarding the Real Estate Professional status.  Real Estate Pros use Schedule C and no safe harbor is used.   But if you don't qualify to be a real estate professional, you use Schedule E, but the safe harbor provides the additional 500 hours needed to qualify for the QBI deduction.  So, if the rentals are on Schedule C, you don't need the safe harbor and if they are on Schedule E, you need it if you want the QBI deduction.   

As far as aggregating - - Taxpayers can treat each rental real estate property as a stand-alone enterprise or group together similar properties and treat each group as an enterprise. Commercial and residential real estate may not be a part of the same rental real estate enterprise. Taxpayers must be consistent with this treatment unless there is a significant change in facts and circumstances.

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