The 27-inch iMac (2015) has a user-accessible RAM door — officially 32GB, some run 64GB

Unlike the 21.5-inch model, the 27-inch iMac (2015) has a RAM access door on the back — no screen removal needed. Officially it supports 32GB (4x8GB 1867MHz DDR3 SO-DIMMs), but some have pushed it to 64GB with third-party modules. Worth trying if you need the headroom.