Children's/YA Science Fiction book- Trapped In Walls?

I am sorry, I have been searching for this book that I only vaguely remember.. It is about a young boy (possibly a girl) that goes up to a manor house, or hotel to work as a servant, but is really there to find a relative, a cousin maybe, that is trapped in the walls, which are really other dimensions. I had thought that there was also something to do with stairs? I think it takes place a long time ago, but was written probably within the last 15 years. The exact details may be off, and that might be why I am not finding it. Any help would be appreciated!

Perhaps Conrad's Fate?

The Woman in the Wall?

Behind the Attic Wall?

Conrad's Fate matches the description best so far.

YES! Conrad's Fate, thank you so much, you have no idea- that has been driving me mad. This is the main reason why I subscribe to Librarything.. So I can keep track of what I read- my memory is horrendous. Thank you again!