Every town starts with its home block and from this time on the town protects the land within 300 blocks each direction from it. This land is the "town property". This town property extends with the growing of the town. Not only by claiming more plots but also by showing their obvious will to extend in that direction (roads/marked future plots etc). So from the current "marked" town area it is additional 300 blocks in each direction.
**NOTE: Towns that are members of a Kingdom have this protection extended to 500 blocks.
If anyone builds within this area without allowance, try first to have a reasonable talk to the builder. If that does not work the town is allowed to take down the build and it is up to their goodwill wether or not they give the materials back to the builder. But: The towns has to give proof, that the build has not been there BEFORE the town extended there (screenshot etc). That means if there are no logs of the construction being build, the town cannot expand that way! Same rule applies for town residents, if they build wildly in a town without the explicit allowance in the town rules to build outside their plots. (see under Town Matters for more details about residents and violating town rules)
Any Dei donated to start a town, or to the town by any person becomes property of the town. You do not have the right to ask for that money back.