The increase of the population of the area in the 1920s and 1930s led to a large expansion of the waterworks. The Diesel House was built to house diesel pumping sets to supplement the steam engines. It contains two Ruston sets, one to raise water from a new well under the building and one to supply the service reservoir.
A third pumping set, to transfer water from Twyford to Yew Hill service reservoir, was delivered before the completion of the diesel house. It was placed in its own ‘temporary’ wooden hut where remains to this day.