https://www.historycolorado.org/
The term I-House was first popularized in the 1930s following cultural historian Fred Kniffen’s notation of the common recurrence of this building type in the “I” states (Iowa, Indiana, and Illinois). The earliest examples of the I-House were found in Iowa and its surrounding regions starting in 1840. At that time, in what was then the frontier, the I-House building type was most prevalent in rural areas. This building form moved westward with the expansion of the railroad and the arrival of pioneers (most notably from the Midwest) who came to settle the American West.
Derived from the hall-and-parlor plan, the I-House building type is clearly defined by its layout. It is two rooms wide, one room deep and is separated by a central passage. The I-House usually was constructed with a multiple bay façade fronting the road, giving the appearance of a larger house. Ambitious homeowners often added simple stylistic elements such as Victorian style porches or elaborate chimneys to enhance exterior appeal. Most I-Houses are two story dwellings, but one- and one-half story versions also exist.
https://www.historycolorado.org/
Though typically found in rural locations, the occasional I-House can be spotted in urban areas and small mountain communities of Colorado. While I-Houses in the Midwest most often were constructed of brick, stone or wood, Colorado I-Houses are predominantly wood frame construction. Wood frame construction was prevalent in Colorado because lumber was readily available from railroad shipments. Colorado I-Houses also differ from those in the Midwest in terms of chimney position: in the Midwest the chimney is usually in the gable end but in Colorado most chimneys are centrally positioned. The majority of I-Houses in Colorado were built between 1875 and 1910.
What makes a house an I-House?
Common elements of I-House: rectangular plan, two rooms wide and one room deep, gabled roof, central passage, lack of ornamentation.
Where to look
Few intact examples of the I-House exist in Colorado, with many buildings of this form featuring one story additions on the rear. These additions usually do not match the original construction but instead reflect a popular architectural style from when they were constructed. One of I-House style home you can see on South Josephine Street in Denver.
Source:
https://www.historycolorado.org/
https://www.aplaceformom.com/