Japanese ready-built houses are usually located in a line across a narrow gap. And the same poor design are repeated there, and a tedious townscape is made as the result. We paid attention to the gap, and inserted an interface that gives precious externality in high-density area into such a mere dead space. Concretely, this project is that two small ready-built houses which often exist in Japanese high-density residential area are united by Gap Union that is newly coined word. Generally, Japanese small ready-built houses are short-lived. Gap Union is a new renovation system and a method of opening a new possibility in the future of such existing ready-built houses. It gives such a townscape an accent, and induces change of the townscape.