The oldest surviving building in the two Columbias is the house of merchant and cotton planter Ammon Underwood, who came to Velasco in Mexican Texas in 1834 from Boston. It originally faced the Brazos River (East Columbia's real main street), just downstream from what had been East Columbia's Front Street commercial district. In 1838 Underwood bought what was a three-year-old, two-room frame house in partnership with widow Catherine Carson. He and Carson expanded it to its present appearance: a two-story, central-hall-plan house with a two-story front veranda incorporated beneath the side-gabled roof. The portion of the house to the left of the front door is the oldest part. In the 1860s, again around 1900, and in 1937 the house was moved on its lot as the eroding riverbank threatened to undermine it. During the c. 1900 moving, the house was rotated to face Main Street. The Underwood House was rehabilitated in 1975 by the First Capitol Historical Foundation for use as a historic house museum.
To the south of the Underwood House, also facing Main Street, is a side-gabled, single-pen log cabin with V-notched corner joints and an attached shed-roofed porch. The cabin is dated to about 1850 and was moved to this site by the First Capitol Historical Foundation from the John Sweeny Plantation in western Brazoria County. It was one of the slave houses on Sweeny's plantation. After the Civil War, it was home to Mark and Larkin Waddy, formerly enslaved people whose descendants occupied it as a tenant farm cottage until 1953.