Christopher Tolkien's preface to History of Middle-earth 4 - the Shaping of Middle-earth provides the answer:
This book brings the 'History of Middle-earth' to some time in the 1930s: the cosmographical work Ambarkanta and the earliest Annals of Valinor and Annals of Beleriand, while later than the Quenta Noldorinwa - the 'Silmarillion' version that was written, as I believe, in 1930 - cannot themselves be more precisely dated.
This is the stage at which my father had arrived when The Hobbit was written. Comparison of the Quenta with the published Silmarillion will show that the essential character of the work was now fully in being; in the shape and fall of sentences, even of whole passages, the one is constantly echoed in the other; and yet the published Silmarillion is between three and four times as long.
So therefore the works that were in existence at the time that the Hobbit was written were:
- The Book of Lost Tales (this is the "Mythology for England")
- The Lay of the Children of Húrin
- The first version of the Lay of Leithian
- The Sketch of the Mythology
- The Quenta Noldorinwa (first major version of the Silmarillion)
- The Earliest Annals of Valinor and Beleriand
- The first Silmarillion map
Of course, the version of The Hobbit that Tolkien wrote in the early 1930s was not the very same as that which was published, and subsequent revisions occurred between then and it's publication date of 1937. During this time Tolkien also wrote:
- The first versions of the Fall of Númenor and the Last Alliance (including Mordor), bringing in the Second Age
- The Lost Road
- The Ambarkanta, "Of the Fashion of the World", (this was included in HoME 4 but Christopher Tolkien dates it to the later period)
- The Second Annals of Valinor and Beleriand
- The Ainulindalë
- Quenta Silmarillion (1937 version, that was submitted for publication as a potential sequel to the Hobbit, but rejected, leading to the writing of Lord of the Rings)
- The Lhammas, "Account of Tongues"
Elements in the Hobbit that were not new inventions but had already existed in these works include:
- The Necromancer (who was always intended to be Sauron) and Mirkwood
- The Three Kindreds of Elves
- Goblins (i.e Orcs) and Dwarves
- The High Elves journey to Valinor and the return of the Noldor
- "The Goblin Wars" (from context these are clearly the Nírnaeth Arnoediad)
- Elrond and the "heroes of the North"
- Dragons and werewolves
- The Ruin of Doriath
The only major post-Hobbit additions to the legendarium were:
- The Rings of Power
- The Third Age
- The Events of the War of the Ring
Although of course all of the previously existing work was modified and (sometimes substantially) expanded, the basic structure and outline of the work remained quite fixed from the pre-Hobbit years.
As Christopher Tolkien says in his foreword to the Silmarillion:
In all that time The Silmarillion, considered simply as a large narrative structure, underwent relatively little radical change; it became long ago a fixed tradition, and background to later writings.