These two stamps are on the south side of Prospect Street between Virginia and Rosamond. One is on the driveway of a house and the other is on its front walk. (There is inexplicably no sidewalk on the south side of Prospect between Holmes and Clifford.)
I can’t find anything out about Washburn Construction. I can find references to a Washburn Construction in Shelby (which is on the west side of the state between Whitehall and Ludington) but that is too far-flung to be likely. There is a Washburn Contracting Innovations in Almont (in the thumb region), but they appear to focus on carpentry. I doubt this was either of them.
The house itself (built in 1924) is extremely cute. This style of house, with the rounded roof and eyebrow dormers, isn’t common in the neighborhood. In fact, I don’t know of another one like it.