By 1908 most houses were being built with indoor plumbing of some kind. I don't think indoor toilets were at all unusual. The fact that outdoor toilets in cities persisted until after the war was largely due to older, poorer slums.
Where conversions have happened, it's pretty variable depending on the type of house, and often you'll find that the original structure of the house has changed completely. The kitchen, which was usually just the range in the main living room of poorer houses, has often been separated. In larger houses servants quarters have been brought into the main house, and the house may have been subdivided into flats. In poorer houses various subdivisions will have been joined into single houses or flats. Cold presses, where food was stored away from heat, and coal bunkers have been converted to utilities, toilets and bathrooms. Some houses will have had sculleries, which is halfway to a bathroom anyway.