If agreement can be reached between the parents of a child as to where that child should live there is no need to ask the family court to become involved. A child can live with both parents at separate times under a shared living arrangement. A legal order may be sought to underpin this agreement.
The family court will operate in accordance with the general principle that delay in determining with whom a child should live is likely to prejudice the welfare of that child (section1(2) Children Act 1989).
When the family court is asked to make a decision about with whom a child is to live (previously called custody or residence) the welfare of that child is the court’s paramount consideration.
When determining what arrangements would best promote the welfare of a child, the court shall consider a number of factors, referred to as the welfare checklist (in section 1(3) of the Children Act 1989). The factors include:
The House of Lords through the opinion of Baroness Hale identified at least three circumstances in which a person may be or can become a natural parent of a child, each of which may be a very significant factor in the child’s welfare depending on the circumstances of the case:
The fact that one parent is the biological parent of a child and the other is not does not lead to a presumption that the child should live with the biological parent. However, the court will likely consider it an important and significant factor, amongst many others, in determining how a child’s welfare is best met, now and in the future (Re G  UKHL 43) and B (A Child) (Residence: Second Appeal)  UKSC 5).
Cases where a same sex female couple have conceived using a known sperm donor
The Court of Appeal in A v B and C (Lesbian Co-Parents: Role of Father)  EWCA Civ 285 declined to endorse the concept of ‘primary' and ‘secondary' parenting, holding that these terms risked demeaning a biological parent who could play a valuable role in the child's life. The Court of Appeal emphasised that cases where the biological father is not an anonymous sperm donor fall within a very broad spectrum.
That meant that such cases are very fact-specific and any general guidance would be inappropriate. The child's welfare was always paramount and this would dictate the outcome in each case; neither the intention of the parties when the child was conceived, nor the feelings of the adults, should be elevated beyond any other factor relevant to the child's welfare.
Arrangements for modern families can be complex. If concerned or in doubt, specialist early legal advice can assist in obtaining the best outcome.