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Watts lived with the Prinseps at Little Holland House, the dower house on the Holland Estate (photo and 1863 map), which was demolished in 1875 to allow Melbury Road to be laid out. Watts then had New Little Holland House constructed and moved in in 1876. There he lived and worked until his death.

New Little Holland House was designed for Watts by Frederick Pepys Cockerell, with a gallery extension by George Aitchison added in 1878. It was demolished in 1964, being replaced by a block of flats (Kingfisher House) c.1965. 

This photo from Layers of London is captioned there as "G.F. Watts plaque, 6 Melbury Road, 1963" so it shows New Little Holland House just prior to its demolition. The '6' in the caption suggests that the road has been renumbered, since the current 6 is nowhere near Kingfisher House.

Sources include: Wikipedia, Layers of London, Artists at Home, Wikipedia.