"if the house is rented and owned by a housing association, SOMETIMES they do have rules and regulations in place concerned with how many children, and which sex of children, can sleep in any one bedroom. " Taken form Should Children of the Opposite Sex Share a Bedroom? which is easily found using google
You will need to check with your local HA or council for their rules.
When I was on the housing list with 1 child and only eligible for 2 bedrooms, I fell pregnant with my second. The eldest was 15 at the time and once I had had the baby I called to let them know about the extra child and because of the ages of the children I was then entitled to 3 bedrooms - both were girls. Moral of my story? Each HA and/or council will have their own rules and regs