First order of business when I get in will be what I know as a measured survey; out with the measuring tapes, pencil and drawing board to produce a series of drawings of the structure in its current state. I can foresee myself producing a few sets of these, one purely structural, one for services, lighting, electrical outlets, waterpipes etc etc etc. Then a trip to the local records office to find any information about the house as originally built; one document lists it as built in 1900, another lists it as 1920, the local historical society place it 1900-1910 and the Ordnance Survey doesn't show it before 1923.
When that's done I'll do some small scale investigation work to find any remnants of previous decor or signs of any panelling or railing that has been stripped out (it would surprise me if I find anything dating back to the time of construction, the walls must have been replastered at some point in the last 100 years). Then I can start drawing up my own plans.