Returning interface but not knowing the concrete type

[ghost]

I have a question about how to handle returning an interface type without knowing the exact type.
Suppose I have a Room service with this schema:

type Query {
  room(id: ID!): Room @merge(keyField: "id")
}

interface Room {
  id: ID!
  name: String!
}

type Kitchen implements Room {
  id: ID!
  name: String!
  sinkCapacity: Int!
}

type Bedroom implements Room {
  id: ID!
  name: String!
  bedCount: Int!
}

And then a house service:

type Query {
  house(id: ID!): House
}

interface Room {
  id: ID!
}

type House {
  id: ID!
  rooms: [Room]
}

But the house service does not know the type of the room. It only knows the room ID.

Is it possible to stitch these two graphs together? And what type should the house subgraph return for rooms?