Castle is a single-family home in a quiet residential neighborhood of varying typology in Hillsborough. To respect the quaint character of the street, we proposed horizontal expansion below the existing roof ridge farther away from the street. On the side not visible from the street, a modern design language with flat roofs with an open concept is introduced; a new rectangular form is juxtaposed against the California ranch-style home, and one big gesture of a horizontal line ties everything together cohesively. This line creates the design datum, provides shade and weather protection for outdoor space, and finally becomes a roof allowing the building volumes to step down along the site’s natural sloped topography gradually. The primary bathroom is a place of rejuvenation inspired by the Japanese outdoor onsen bathing custom that borrows the outdoor natural elements as part of the serene bathing experience.