How common the arrangement is, with the denominators stated. Using 2020 Census data, the Bureau reported that multigenerational households "made up 4.7% of all U.S. households but 7.2% of family households in 2020, an increase from 2010," and counted 6.0 million such households in 2020 against 5.1 million in 2010. County shares of family households ranged from 0.5% to 31.0%. Those two percentages are the ones to be careful with, because 7.2% is a share of family households only and is frequently reprinted as though it were a share of all households. Both are 2020 measurements; a current figure comes from the American Community Survey's own table on household type rather than from a secondary summary.
A program's household is defined by the program. This is the fact the page exists to carry, and it is where multigenerational households encounter the most avoidable trouble. Food benefits are administered to a household defined by the food benefit rules, which turn on how food is bought and prepared rather than on the address, and which contain a specific route for an elderly or disabled member to be treated separately. Federal income tax has no concept of a household at all: it has filing statuses, dependents and a support test, each with its own criteria, and head of household in particular is a filing status with its own conditions rather than a description of who lives where. Health coverage subsidies use the tax household, which follows the tax return rather than the residence. Medicaid eligibility is determined by rules that differ by state and by category, and a transfer of a home to a caregiving child is an express exception to the transfer penalty rather than an ordinary gift. Each of those has its own page here, and the point that generalizes is that answering "are we one household?" requires naming which program is asking.
Buying or building together: the three ways a parent's money can arrive. Where generations pool resources into a home, the same dollars can be a gift, a loan or an equity interest, and the choice is made by documentation rather than by intention. As a gift, the money buys the recipient's equity, the giver owns nothing, and an amount above the annual gift tax exclusion of $19,000 per recipient requires a gift tax return even where no tax is due. As a loan, it has to be documented and carry interest at or above the applicable federal rate, or the forgone interest is treated as a gift anyway. As an equity interest, the contributing generation goes on the deed and owns a stated share, which is the only one of the three that survives a later sale, a death or a divorce as a claim on the property. Whose name is on the mortgage is a separate question from whose name is on the deed, and the deduction for mortgage interest generally follows the person who both owns the home and is obligated on the debt, rather than whoever writes the monthly check.
What co-ownership exposes each generation to. An owner's share of a property is reachable by that owner's creditors and is part of that owner's estate, so putting a parent on the deed brings the parent's medical creditors, and the parent's eventual estate administration, into the house. It also fixes the tax basis question: property inherited at death generally receives a basis adjustment while property given during life generally does not, so the form of the transfer decides the capital gain a later sale produces. And the form of co-ownership matters as much as the fact of it, because tenancy in common and joint tenancy with right of survivorship send a deceased owner's share to completely different people.
Space, and the reason it usually gets built. An accessory dwelling unit, the separate living space sometimes called an in-law unit, is the common physical response to a three-generation household, and it carries its own permitting, financing and property tax consequences. It also raises the household question in a sharper form, because a genuinely separate unit with its own kitchen and entrance can change the answer some programs give.
