I currently claim my domestic partner as a dependent on my tax return. Can I claim her daughter if I am supporting both of them as head of household?

[Bsch4477]

Yes, if they live with you all year and you provide more than half of their support AND their taxable income is less than $4,400 (for 2022)  they can be your dependents. BUT only a blood relative can qualify you as Head of household so you would have to file as single. 

[DoninGA]

If you are not married to your partner and you are not related to the child then you cannot have Head of Household filing status.  You can only file as Single.

To claim your partner and the child as dependents they must meet all the Qualifying Relative rules.

To be a Qualifying Relative -

1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.
2. The person either (a) must be related to you or (b) must live with you all year as a member of your household.
3. The person's gross income for the year must be less than $4,700 (social security does not count) in 2023
4. You must provide more than half of the person's total support for the year.
5. The person must be a U.S. citizen or a U.S., Canada, or Mexico resident for some part of the year.
6. The person must not file a joint return with their spouse.

[Opus 17]

Yes and no.

You can review the rules in publication 501.

https://www.irs.gov/pub/irs-pdf/p501.pdf

Your DP can only be your dependent if they live in your home the entire year (as a member of your household), and you provide more than half their support, and they have less than $4700 of taxable income.

The rules for claiming their child are the same for residency, income and support.  In addition, if another taxpayer qualifies to claim them, you can't, even if the taxpayer who could claim them agrees not to claim them.  In the case you describe, the biological parent could claim them, but the biological parent is not a taxpayer as long as their income is less than $13,850 and they either file no tax return, or they only file to claim a refund of withholding and claim no other dependents, credits or deductions.  Since you can't claim the DP unless their income is less than $4700, the DP is probably not a "taxpayer", and that will allow you to claim their child.

However, an unrelated child only qualifies as an "other dependent" for a $500 credit, not the $2000 child tax credit.  And neither the DP nor their child is a "qualifying person" for head of household status, so you have to file as single. 

[Scmeyer]

This is really good content and informative. To provide a little more detail, my domestic partner and I have 3 children together that are biologically ours to qualify me as head of household. I believe that based on interpretation of the tax law, I was under the same impression that I could claim her biological child (not mine) but only as a dependent for the $500 credit. I believe you confirmed that to be your understanding as well. Thank you.

[Opus 17]

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@Scmeyer wrote:

This is really good content and informative. To provide a little more detail, my domestic partner and I have 3 children together that are biologically ours to qualify me as head of household. I believe that based on interpretation of the tax law, I was under the same impression that I could claim her biological child (not mine) but only as a dependent for the $500 credit. I believe you confirmed that to be your understanding as well. Thank you.

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Yes, if you have other children together, that will qualify you for HOH.  You seem to be on the right track.