It works the same in many parts of Europe,
You count the number of rooms excluding the kitchen, so 2 Peicw would be 1 bedroom and one living room,
The half comes from rooms which are part of the kitchen, so if you had 1 bedroom, 1 living room and 1 separate dinning room it would be 3 rooms, but when the dinning room is part of the kitchen then it becomes 2.5

Hope this helps