All the scenes in the house (but not the attic ones, and the library sequence) have been filmed in the same house with natural light only. The three different stairways existed as such and have been kept for filming.
This is Sergio G. Sánchez's directorial debut. However, he has previously written for films such as The Impossible (2012) and The Orphanage (2007), both Directed by J.A. Bayona, who is also a producer of this film.
In the film, George MacKay plays the oldest sibling, Mia Goth is the second oldest, Charlie Heaton is the third oldest and Matthew Stagg is the youngest. This is true in real life with George being born in 1992, Mia in 1993, Charlie in 1994 and Matthew in 2009.