Who should pay the council tax?

I think I'd be rethinking the whole idea and the strength of this relationship with this commitment. If these issues are already a big deal then this won't end well.

I guess the biggest question is what has A said about it all???

Oh ok, sorry. If I were in that person's position, and I wanted to retain the love and respect of my partner, I would unquestioningly pay my half of the council tax from my benefits. I do understand that they're exempt at present. The fact that there's even this question arising doesn't bode well for the joint household in my opinion.

Because they decide to live together as a household. If they want to live separately with separate finances, they shouldn't move in together.

You say you are losing £800 but you are also gaining £700 so the difference is £100, divide that by 3 and ask for £735 ish.
Then all is square!
You are also gaining help with all other bills, don't be greedy.

But they won't be using half, they'll be supplementing the child too

Why 50/50? Why should A be paying 50% of the child’s costs? Their own non resident parent should be paying towards their needs.

Making a joint claim because your boyfriend moves in is a significant change of circumstances and if you were on the protected enhanced payment from transfer to UC from ESA, it will be a new claim.
A couple shouldn't be trying to manipulate benefits.

If it's not a boyfriend moving in and it's a "lodger" your benefits wouldn't change and you'd be responsible for the CT as it's your house.

I agree.

If you aren't both on board with one pot and equal spending, if it's an issue already, it may go on to be a problem. You're quite obviously Person B, which I'd argue makes you more vulnerable, so think carefully.

Why 50/50 when person B has a child who will use facilities?

In most (but not all) areas, the only 100% exemption for disability is if the GP has given a Severe Mental Impairment certificate. A quick Google suggests that there are some LAs such as Camden, Wandsworth, Mid Devon, etc., who do give 100% reduction for being in receipt of PIP, but they go above the requirements of the law.

All bills should be shared 50/50, unless significant discrepancy in income. If so then it should shared proportionally to incomes.
The split should ensure each person keeps a reasonable and fair amount for their own personal expenses.

Everything else is unnecessarily complicated. The loss of benefits is a choice person B makes when their partner move in. Same for the council tax - it's a household, not a flat share. The entire system is based on the fact partners support each other financially (I'm not saying in agree with this, it's just the reality of the benefits and tax system in the UK).

It could be that moving in together is not the right decision for person A or B, financially. It's a choice they both have to make.

To be fair to @Cavapoo123 I don't think she's saying that she should still get a Council Tax Reduction. She's just asking who should meet the cost of the shortfall.

Why are you talking about people being severely mentally impaired? Sorry, you've lost me!

Person B, I assume you, would be better off continuing to live on their own with their DC. I can’t imagine having less money for my child to accommodate living with a partner.

It would be different of you were looking at combined household finances and equal disposable income etc.

You'd think so, wouldn't you? It never ceases to amaze me how the taxpayers can be expected to keep subsidising to such a large extent. The difference now is huge between working or not, especially when you factor in council tax to be paid if you're on average income. Our bill has just come in, and it's £3k per annum.

The OP is confusing since it says both that Person B has no kids, and then that they have a disabled child. Also says that Person B doesn’t work, but then say that they “earn £2000 a month”.

All of that aside, since the council tax arises as a result of Person A moving in, and Person A will be reducing their accommodation costs by half, clearly Person A pays the council tax.

Person A should pay for the Council Tax, because without them in the house there is an exemption in place. If person B and person A can't work this out through an amicable conversation they should not be moving in together.

You wrote "person B has no children", presumably accidentally, so you need to change it.

You don't move in together if this is even a question OP. If you rely on benefits then someone working full time will have to take the load. If they're not completely on board then don't do it.

Resentment has a way of setting in.

Person B should not move in with person A.
They should protect and keep what they have and not risk it.

This is so garbled as to be pretty much incomprehensible. I recommend putting it all on a spreadsheet, with "before" and "after" columns. And if you're arguing about this now, then it will get worse and my advice to Person A (if I've got it the right way round) would be stick to living on your own and to Person B, "Run!"

but you are two @Cavapoo123 your child and yourself, therefore bills should be split accordingly. You have after all been paying it all by yourself for you and your child up until now.

Your partner should not have to subsidise your child (whom you've always been paying for) the tricky part, is, indeed, your council tax.

If you will lose all the reduction, that you've, thus far had, it would not be unreasonable for your partner to absorb your council tax hit.

BUT they should only be paying one third of your household bills.

"Am assuming that the council tax bill for a in their own flat with single discounts in their previous property will now be considerably more with no single discount, different area and bigger property?"

Person A will still get single person discount.

Person B needs to think about whether he wants to be financially responsible for a non-working adult and a disabled child, doesn’t he?

Realistically, the country is unlikely to continue paying so many benefits indefinitely.