Am i right in thinking that if you are in receipt of JSAC and not JSAIB that you cannot start the waiting period for help with housing costs until you go onto JSAIB even if your income is low enough for JSAIB before your entitlement runs out?

Others may know better, but my understanding is that you can apply when you claim CBJSA and the 13 week period starts then. Once the 13 weeks are up, you will still be on CBJSA but will start getting the housing costs.

I agree. The starting point is to calculate the applicable amount which includes housing costs. If the AA is higher than income then the claimant is entitled to IBJSA housing costs. They are simply not payable until the waiting period is served but the clock starts from the date of claim.