11/34 on ultegra 6800?
If...(and this is a BIG if) it's the same exact derailleur as the one on this link. https://bike.shimano.com/en-EU/product/component/ultegra-r8000/RD-R8000-GS.html And your bike uses an 11 speed cassette. Things change year-to-year. Shimano states it covers: 11-28T to 11-34T. The RD-R8000-SS only covers 11-25T to 11-30T. https://bike.shimano.com/en-EU/product/component/ultegra-r8000/RD-R8000-SS.html Those are this year's derailleur models. Who knows...in 2016 it could've been different. So don't take my word for it. I'd double check with a certified bike mechanic at your LBS.
It might work or it might not. Shimano lists the 6800 GS derailleur as having a maximum capacity of 32 teeth for the lowest gear. The RD 8000 GS rear derailleur has a maximum 34 tooth capacity. Shimano tends to be a bit conservative about maximum tooth capacity, so your 6800 derailleur may well be able to handle 34 teeth. If your 6800 derailleur isn't able to deal with a 34 tooth low gear, you simply have to switch to a RD 8000 GS derailleur to handle the larger rear cog