National Register of Historic Places in Butte County
We have been unable to find much documentation for the Allen-Sommer-Gage House. The digitized NRHP nomination has no narrative, just a photographs and a vintage map of Chico.
The Allen-Sommer-Gage House is located within the South of Campus Historic District, but the district nomination contains a single sentence about the house: one of Chico's oldest documented houses, it is in the Greek Revival style. No date is given.
The ChicoWiki website contains some information:
Between the Love Boat and the Pi Beta Phi house on Normal.
The Allen-Sommer-Gage House (also called the Gage House) is a house in the South Campus Neighborhood which was listed in the National Register of Historic Places as an individual property in 1977; and also listed as a contributor to the South Campus Historic District in 1991.
It was designed by R.H. Allen in Greek Revival style and constructed in 1880. It is the oldest house still standing in Chico. It was listed on the Register of Historic Places on April 13, 1977 with ID Number 77000288.
Helen Sommer-Gage was born in the house in 1888 and was living there at the time she died in 1980. As a child she knew John Bidwell.
John Gallardo of the Chico Heritage Association says that the house was built in 1862 (ChicoER.com 15 May 2009), and we will go with this date. If the Allen-Sommer-Gage House had been built as late as 1880, it would not be one of Chico's oldest houses.