Is it actually possible? I know you can disable borders to get there but I want to know if you can build there in the cs, 'cos the land in cyrodiil doesn't cut the cake for what I'm trying to build.
Thanks in advance.

I dont get you. Do you want to build a totally new land, or to build outside the borders. The border regions have lower poly distant meshes and so up close look awful. You would have to disable the border in regions in the CS then build on the land, also replacing distant stuff, however re running the distant land stuff can be tricky.

Might help to describe briefly what you're trying to build.

The Karithian: All I'm trying to do is build outside the borders, but I can kind of see that this will be tricky.
Vagrant0: All I'm doing is building a city. Where there's a few NPC's, shops, etc. Though I'm not expecting it to be the best.

Building just somewhat outside Cyrodiils border is easy, as the cells already exists in the CS. There are numerous mods that do this: Several of the UL mods, several settlement mods, but most noticeably Elseweir which adds most of a new land.