How do building privileges and build zones work?
Every TC creates a build-zone around the structural elements of the building it's in. Inside that zone:
Only players authorized by the TC can place new structures, doors, deployables, or another TC
Unauthorized players will see "Building Blocked" when trying to place
Decay is paused as long as the TC is fed with resources
The zone follows the structural shape of the building - not a fixed radius. Each foundation, wall, or floor that belongs to the TC's building extends the zone around itself.
If your building splits, each separated part keeps its own build-zone only if it has its own TC. Parts without a TC are unprotected - decay starts, and others can build nearby.
→ See also: Why did my base split into multiple buildings? · Why can't I place a Tool Cupboard?