Sex and family / X2
Three-adult household
Three consenting adults want to live together as a family, share property, and make binding arrangements for inheritance, medical decisions and parenting.
What we score
Can they do so lawfully, and can they make their arrangements binding?
Default assumptions: Unless a vignette says otherwise, the person is a 30-year-old citizen of sound mind with no criminal record, acting alone, on their own behalf and not for profit, in an ordinary residential area. Where subnational units (states, provinces, cantons, Länder) make their own law, each unit is scored and the result is weighted by population. 'The state' includes police, prosecutors, courts, regulators, tax authorities and anyone acting on their orders, including platforms obeying takedown orders.
Score anchors
Both the law score and the practice score use these anchors. Half points are used when a country sits between two of them.
| Score | What it means for this vignette |
|---|---|
| 10 | Can marry or register as a group, or make fully binding contracts covering all of it. |
| 8 | Cohabitation free; contracts, wills and powers of attorney cover most of it. |
| 6 | Cohabitation free; some arrangements (parenthood, inheritance) cannot be made binding. |
| 3 | Cohabitation itself attracts administrative penalties. |
| 2 | Cohabitation is a crime (bigamy or cohabitation laws enforced): fine or under 1 year. |
| 1 | Crime: 1 to 5 years. |
| 0 | Over 5 years, or corporal punishment. |
Evidence used: Bigamy and cohabitation laws as applied; enforceability of multi-party contracts; parentage law.
How each country scores
| Country | Law | Practice | Process | Score | Why |
|---|---|---|---|---|---|
| United States | 7 | 7.5 | 0 | 7.3 | Three adults living together commit no offence. Bigamy laws apply only to purporting to marry a second spouse, and Utah cut even that to an infraction for consenting adults in 2020. Wills, powers of attorney, health-care proxies, co-ownership deeds and cohabitation contracts cover inheritance, medical decisions and property. Group marriage is unavailable. A third legal parent can be recognised by courts only in a few states such as CA, WA, ME, VT, DE, CT and CO, so parenthood often cannot be made binding. |
| United Kingdom | 7 | 7 | 0 | 7.0 | Cohabitation by three adults is lawful (bigamy applies only to a second marriage). Property, wills (E&W has testamentary freedom) and lasting powers of attorney can cover most arrangements. Only two people can be a child's legal parents, though a third adult can get parental responsibility through a child arrangements order (Children Act 1989 s12), which excludes consenting to adoption or appointing guardians. |
| New Zealand | 7 | 7 | 0 | 7.0 | Cohabitation by three adults is lawful; bigamy (Crimes Act s205) applies only to a second marriage or civil union. Contracts, wills and enduring powers of attorney cover most arrangements, though Family Protection Act claims can override wills. A child has at most two legal parents, but a parent's partner can be appointed an additional guardian, and courts can appoint guardians (Care of Children Act 2004), so a third adult can share legal parenting responsibilities. |
| Ireland | 7 | 7 | 0 | 7.0 | Cohabitation of three adults is lawful; bigamy applies only to a second marriage. Property, wills (only spouses and civil partners have forced shares) and enduring powers of attorney can cover most arrangements. A child can have at most two legal parents, but under Guardianship of Infants Act 1964 s6C a non-parent who has cohabited with a parent for over 3 years and shared the child's care for over 2 years can apply to the court to be appointed guardian. |
| Czechia | 6 | 6 | 0 | 6.0 | Only concluding a second marriage or registered partnership is a crime (§ 194). Cohabitation of three adults is lawful. Wills, co-ownership contracts and healthcare proxies are available under the Civil Code. The law recognises only two legal parents and no group marriage or partnership, so parenthood and spousal inheritance cannot be extended to a third adult. |
| Switzerland | 6 | 6 | 0 | 6.0 | Living together is unrestricted; bigamy (Art. 215) covers only a second marriage or registered partnership. Contracts, wills (within reduced forced-heirship shares) and advance care directives cover property and medical decisions. Legal parentage is limited to two parents (Civil Code Art. 252), so a third adult cannot become a legal parent. Cohabitants also pay much higher cantonal inheritance tax than spouses. |
| Netherlands | 6 | 6 | 0 | 6.0 | Cohabitation by three adults is free, and cohabitation contracts, wills and medical powers of attorney can cover property and much of inheritance (subject to children's forced share). Bigamy (art. 237 Sr) applies only to entering a second marriage. Current law allows at most two legal parents and two holders of parental authority. A government draft bill allowing up to four legal parents was in consultation until 31 January 2026 and has not been enacted. |
| Mexico | 6 | 6 | 0 | 6.0 | Three adults may live together lawfully. Bigamy under Federal Penal Code art. 279 (up to 5 years) applies only to contracting a second formal marriage. Wills, property contracts and powers of attorney can cover most arrangements, but civil codes allow only two legal parents and marriage only between two people, so joint parenthood and spousal inheritance for all three cannot be made binding. |
| Germany | 6 | 6 | 0 | 6.0 | Three adults may live together freely. Bigamy (§ 172 StGB, up to 3 years) applies only to a second marriage or civil partnership. Wills, co-ownership contracts and healthcare powers of attorney work, subject to forced-heir shares. A child can have only one mother and one legal father (§§ 1591-1592 BGB), so third-parent status cannot be made binding. |
| Singapore | 6 | 6 | 0 | 6.0 | Cohabitation is not regulated. Civil marriage under the Women's Charter is monogamous, and bigamy applies only to a second marriage. Non-Muslims can use wills and Lasting Powers of Attorney to arrange inheritance and medical decisions among three adults, but the law recognises at most two legal parents, and Muslim estates follow faraid rules that limit bequests. |
| Denmark | 6 | 6 | 0 | 6.0 | Bigamy (§ 208) covers only entering a second marriage or registered partnership; three adults may live together and share property freely. Wills, co-ownership agreements and lasting powers of attorney cover most arrangements, but a child can have at most two legal parents, and forced heirship gives children a reserved share of an estate. |