Three cutaway sections through 165 Eaton Place. Click on the images for enlargements. The first cross section was originally published in the TV Times; the second in tvlife, and the third in the 1972 TV Times special. Click on each for a larger version. Although they bear a superficial resemblance to what we saw on the TV screen, attempts at accuracy soon crumble on close inspection (e.g. the kitchen range is against the wrong wall in the first diagram etc.)
In the TV show, the layout of the bottom three storeys remained fairly consistent. The basement was occupied by the kitchen, servants' dining room/hall, (two?) sculleries (see, for example, I Dies from Love and A Patriotic Offering), coal house (see Rose's Pigeon), Mr Hudson's pantry (later, apparently, his bedroom too), a walk-in larder, and a wine cellar (see Your Obedient Servant). Towards the third season, it is also implied that Edward's bedroom is in the basement, too.
The ground floor had the hall, morning room, study and dining room (which grew in size for later episodes – compare A Suitable Marriage with Guest of Honour for example). A library was also mentioned as being on this floor (also in Guest of Honour) but was never seen. The early episode The Swedish Tiger presented a different layout for the ground floor and included an octagonal gaming room where the study would be later shown to be.
The first floor was almost entirely occupied by the drawing room (which grew substantially in size as the show progressed, reaching its climax at around the fourth season).
Rooms on floors above this were only shown intermittently and there seemed to be no real "official" layout other than the general idea that the servants' bedrooms were at the very top of the house. (This is because the production team would only erect the sets they needed for a particular episode and there was little thought given to how the lesser-used ones fitted together.)

Waiting to shoot an exterior shot of the house.

Producer John Hawkesworth (left) and script editor Alfred Shaughnessy (middle) oversee location work in Eaton Place.

The Bellamys' Rolls Royce outside (1)65 Eaton Place. Note the box of props occupying the passenger seat.

A picture postcard of Eaton Place. It is undated but is likely from around the turn of the century.