So I am upgrading my Trek 1.1 (2016) from 8speed Shimano Claris to an assortment of Shimano Ultegra 6800, 105 5800. I just bought a 11speed cassette to complete the major parts (it hasn't shipped yet). Will I also have to buy a larger free hub assembly so that the new cassette will fit? I've never measured the difference so I'm not sure if its necessary.
As can you see here, for every increase in cog numbers there is also a increase in freehub width. So yeah, you will have to find another hub or freehub body. Changing only the freehub assembly depends on the model you have, but in general it is possible. Since your bike is kinda lower end (no offense), I would say you should do the math to see if it is actually worth it, instead of simply buying a new, better hub.
Does the wheel dish change as well?
"Shimano 11 speed road cassettes require wider freehub body. You have several options here:
Get a new rear wheel or hub that supports 11 speed road cassettes. Get a replacement 11 speed freehub body if there are any (most likely not). Get a Campy freehub body and Campy 11 speed cassette instead of 5800. Get this Token cassette. It's an 11 speed road cassette that fits 10s freehub body."