If you aren't planning on including your parents in your tax return meaning if you won't be claiming your parents as a dependent (if you do your income will be counted too and they may end up paying more for their premium) in your tax return then you don't need to count them in your household. Basically, your parents need to submit health insurance application (2 people in the household, your mom and dad only) and need to attest that they will be filing taxes even if they don't need to file based on their income in order for them to receive APTC. Hope this helps.