How do I merge two buildings into one?

To merge two buildings, connect them with structural blocks placed exactly side-to-side - flush contact between foundations, walls, or floors. Once that contact exists, the system treats them as one building.

Important caveats:

• The connection must be a full side-to-side fit. Corner touches or partial overlaps do not merge buildings

• Building two separate structures from scratch with perfect side-to-side alignment is practically impossible - the in-game placement grid won't allow it

• Merging usually only works when restoring something that was once a single building and got split

After the merge, the building has one TC and one build-zone again. The TC of the absorbed part is removed (or stays if there's only one).

→ See also: Why did my base split into multiple buildings? · How does decay work?