I'm moving down south, leaving Northumberland for the first time.
I’m moving into my new property on 17th September. Can I register with the new council now for council tax and Housing Benefit?
According to my local council's website, claims for Housing Benefit and Council Tax Reduction usually start from the Monday following receipt of the claim. So, assuming that a similar policy applies where you'll be moving to, applying during the week beginning 6 September would seem to make sense to me.
Assuming that it's Southampton that you're moving to (based upon your previous posts), I suggest phoning 023 8083 3009 to check. Lines are open from 0830 to 1700, Monday to Friday, except on Wednesday (when the lines open half an hour later, at 0900) and Friday (when they close half an hour earlier, at 1630).
"You can apply for [Council Tax Reduction] up to 8 weeks before you’re first responsible for paying council tax on your home - for example if you haven’t moved in yet."
"You can claim [Housing Benefit] in advance by up to 13 weeks (or 17 weeks if you’re aged 60 or over), for example if you’re moving. You will not usually get any money before you move."