Plots and permissions

Plot ownership

Each player can own a plot of land on which they can build structures. Players with paid accounts will receive one plot, and additional plots may be acquired through other mechanisms.

Claiming a plot

Building can only take place on designated plots of land. Players need to find a suitable location for their plot within the Heartland valleys, as building is not allowed outside these areas. Claiming plots on pre-made roads, underground, or on dungeon entrances is also prohibited. Plots can overlap with water though.
Here's a developer quote in regards to "plot positioning":

You'll get a preview of the borders in the world and can move it about, then confirm when you're happy with the placement.

Build area on a plot

Building access on a plot only allows to use a certain portion of a plot for building, unless there are surrounding plots which one also has building access to. The developer stated that there will be significant benefits/advantages to join plots, not just bigger buildings and less wasted (inaccessible) build area.

Plot size

The final size of a plot is currently not determined yet. Here are some developer quotes on what to expect though:

We haven’t yet determined how large the plots are, so it’s impossible to answer this exactly. The balance we’re looking for could be described as “you can build quite a lot on your plot, but you will also want to work with your clan to use the combined land in your clan’s plots as smartly as possible”.

...this portion of land is circular and vast enough to build a house of reasonable size and a couple of smaller structures.

Shared plots

Plots can be shared among players or within clans. This enables groups of players to collaborate on building projects and manage multiple shared structures across different locations.

Building on others' plots

Players can build on someone else's plot, but only if the plot owner has granted them permission to do so.

Access control

Permissions can be managed to allow certain players or clan members to access or modify structures on the plot. Plot owners can restrict who can access their structures, but they cannot completely block access to their plot via permissions. 
There's an interesting quote in that regard though:

We don’t have a feature to restrict people from walking on your land, but you can build walls, so theoretically, your clan could wall up the village - as people did in the Middle Ages.

Abandoned plots

If a player stops playing the game or becomes inactive for an extended period, their plot may be deemed abandoned. In this case, the game developers plan to free up the plot for other players to occupy, while providing a grace period and potential rebuilding options for returning players.