A "sponsor unit" is an apartment owned by the original developer (or a successor sponsor) that has never been sold to an individual owner. They show up in both co-op and condo buildings, and they come with one big upside — typically no board approval — and one big downside — buyer often pays the transfer taxes the seller would normally cover. Whether a sponsor unit is a good deal depends on three numbers: the price, the closing-cost shift, and how the unit's condition compare