Locating house centre

[myfs_100595]

I am Gua 2, my husband Gua 2(5). We live in a period 7 SE1 house. A number of years ago, the landlord converted the house into two flats by installing a doorway in the middle. So, we enter into a square hallway, facing this door, which leads to a back hallway/staircase that leads in turn to the back door of the building proper. Essentially, these two halls divide the house in two, with bedrooms along corridors to the left (going in, SW side)and sitting room, kitchen, study to the right (NE side). The back hallway is shared with our one upstairs neighbour and as well as being our back entrance has the staircase leading up to his flat.
My question is, do I include this back hallway when I measure the centre of the house, or would the house count as U shaped with this sector missing?
Many thanks,
Marilyn McMurtrie