Standing at the foot of staircase, facing the stairs, you can see the secretary's room diagonally opposite you on the left-hand side. Source: Mindset for IELTS 3, page 65
Having grown up in the south-east of England, but with lots of extended family members from the Midlands (and other places north of Watford), I was very familiar with the phrase. I couldn't understand why all my peers at school were so baffled whenever I used it.