Is this Prospero's Children (1999) by Jan Siegel...? It's the first book in the Fern Capel trilogy.
It began ages past in fabled Atlantis, when a mad, power-hungry queen forged a key to a door never meant to be opened by mortal man--its inception would hasten her own death and the extinction of her vainglorious race. For millennia the key lay forgotten beneath the waves, lost amid the ruins of what had been the most beautiful city on Earth. But however jealously the sea hoards its secrets, sooner or later it yields them up. Now, in present-day Yorkshire, that time has come. And for young Fernanda Capel, life will never be the same again . . .
This review covers most of the plot points you mentioned:
- A teenage girl, Fern Capel, moves into a house with her younger brother, Will, and father, Robin.
- Robin is beguiled by Alison Redmond, a woman Fern suspects of being a witch. Alison stays over at the house while Robin is away on business, and is later discovered looking for a missing key to a doorway in the house.
- It turns out that the doorway in question leads to Atlantis, and Fern must go through it.
- The father, Robin, is said to be a publisher rather than someone in the art world, but the house is visited by Alison's friend, Javier Holt, who owns an art gallery.
Sixteen-year-old Fern Capel lives in London with her younger brother, Will, and her father, Robin, a publisher. When a distant seafaring relative leaves Fern a house in Yorkshire, they all go to stay there for the summer. It's an odd place, with a ship's figurehead in the barn, a mysterious locked, keyless desk, and numerous uncanny goings-on. It doesn't help that Robin's beguiled by Alison Redmond, whom Fern suspects of being a witch. Yet the Capels find friends as well: Ragginbone, who sometimes turns into a rock, mentions a missing key that opens the Gate of Death; and Lougarry, a werewolf unable to resume her human form, helps defend the house against malevolent intruders. Alison, however, invites herself to stay while Robin's away on business. Her friend, the suave, too-clever art-gallery owner Javier Holt, stops by. An evil stone idol casts a pall in the front room. Fern frees a unicorn trapped in a painting in Alison's room. Then one night Fern steals downstairs to find Alison conspiring with the idol to learn the whereabouts of the key. Finally, when Fern locates it, Alison grabs it and flings wide the Gate—but it's a doorway into Atlantis! Seawater pours through, drowning Alison, though then the door doesn't close. Fern, a descendant of Atlantis, must enter. Despite the sometimes suspect logic and overwrought prose, an inventive, well-handled effort, with commendable character development. An encouraging debut—stay tuned for the rest of the trilogy.