Some flip taxes take a percent of profit (vs sales price), which can get tricky if sellers are allowed to subtract the cost of improvements. (A 900-unit co-op in Morningside Heights apparently charges a "whopping" 15% of the net profit.) Other flip taxes go by how many shares are allotted to the apartment, which doesn't account for inflation.
Being notoriously hard to implement or change (usually requiring a 2/3 vote of shareholder), it's important to get the flip tax right the first time.
The magic number, according to a property manager quoted in the Habitat article (not yet available online), is around 2% of the total sales price: "This percentage seems just high enough to make a positive difference for a building's reserve account and just small enough to feel manageable."
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