Council tax benefit backdating regulations
how the applicant’s income and capital is assessed) and does not include the power to exclude or limit entitlement on any other grounds such as residency qualifications or nationality R (Winder and others v Sandwell MBC)  EWHC (Admin) 2617.
Therefore the law relating to CTR entitlement for migrants is found in the prescribed requirements and default scheme regulations (one set of each for each country), as follows: For each set the default scheme regulations (relating to new arrivals) say exactly the same thing as in the prescribed requirements regulations.
So if in a couple one member is eligible for HB/CTR and the other is not the eligible member should make the claim.
Provided the eligible member makes the claim the award is calculated in the normal way and is not adjusted to take account of the fact that one member is ineligible, except that any income and capital the ineligible member possesses is still taken into account together with that of their partner.
This page gives detailed guidance on aspects of the law relating to housing benefit (HB) claims and to council tax rebate (CTR - also known as council tax reductions) claims that are relevant to new arrivals.
In the case of a claim for HB an application for a NINO is sufficient if it is accompanied by all the evidence and information required to process it (section 1(1B)(b)).
We refer throughout to the ‘HB Regulations’ although there are in fact two sets of regulations, one each for pension age and working age claims with relevant law referenced as ‘HB60 ’ or ‘HB’ respectively in both cases, followed by the relevant regulation and paragraph.
CTR law is no longer part of the social security system.
Any matter relating to the provision of a NINO can be appealed to a tribunal, including the evidence that is required for one to be ascertained or the consequences for an award of HB if a request to allocate one is refused: CH/1231/2004 and  UKUT 74 (AAC).
Apart from the requirement to provide a national insurance number (which applies to both members of a couple) the two mechanisms by which migrants and recent arrivals (see the law on entitlement) can be excluded from benefit apply only to the claimant (and not also to their partner).