The Tolkien Ents were sentient trees, so I doubt they would be used for houses. However, they were able to act as shepherds of a sort for the old forests. Now, if you had a sort of guardian tree race, they could direct the growth of the non-sentient (but still very much alive) trees to shape them as houses for your elves.
(Ettins are D&D lore—two-headed giants with severely bad tempers. )