Receiving carers allowance when on HB

Fostering allowances are disregarded income for many means tested benefits.
"Fostered children are not counted as part of your household when any means tested benefits are calculated.
Equally the fees and allowances from fostering are not counted as income when calculating any of the following means tested benefits: Income Support, Income based Job Seekers Allowance, Income based Employment & Support allowance, Housing benefit and Council Tax Support. "


"Carer’s allowance is a benefit paid by the Department for Work and Pensions (DWP). You don’t need to have paid national insurance contributions to get it, and it is not affected by any savings you or your partner have. It is, however, taxable and counts as income for tax credits. It may also count as income for other benefits. It can also be affected by any earnings you have from work (but not your partner’s earnings).

For 2016/17, Carer’s allowance is £62.10 a week. Carer's allowance is a taxable benefit.

You can normally claim carer’s allowance if:

you are aged 16 or over;
You have been in England, Scotland or Wales for at least 2 of the last 3 years and you normally live in England, Scotland or Wales or you live abroad as a member of the armed forces. (The rules for Northern Ireland are different, see the NI Direct website for more information). There are some exceptions to these conditions if you’re living in another European Economic Area country.
You are not subject to immigration control (although there may be some exceptions to this)
you spend at least 35 hours a week looking after someone (this could be a child) who gets the middle or highest rate care component of disability living allowance, a personal independence payment daily living component, either rate of attendance allowance, armed forces independence payment or constant attendance allowance (at or above the normal maximum rate with an Industrial Injuries Disablement Benefit or basic (full day) rate with a War Disablement Pension)
you are not attending a full-time course of study (21 hours or more a week); and
(if you are working) you usually earn no more than £110 a week. This £110 a week is after deducting tax, national insurance, half of any contribution to an occupational or personal pension and certain other expenses. This may include childcare costs and payments you make to other people (not close relatives) to provide care while you work. Any occupational pension you get is ignored, as are certain payments from the council for someone temporarily in your care (e.g. fostering allowances).

As pmlinyloo says CA will be included as income in the HB calculation, but you will be allowed more income (your applicable amount) because you are reciving CA.