The architecture at Colvin Run Mill changed over time, and remnants of modifications made by the Millard family during their more than 50 years in the Miller's House remain visible. An upside-down "V" near the roofline marks the location of a two-story addition the Millards built to accommodate their growing family, probably with bedrooms upstairs and a kitchen and dining area on the first floor. At the back of the house, where a flower bed now sits, the Millards built a one-story addition they called "Papa's room." Addison Millard later used it as his bedroom when he suffered from white lung and could no longer climb stairs. These 19th-century wooden additions to the Miller's House were removed in 1967, when the Fairfax County Park Authority restored the house to its original appearance.