When couples marry they often bring to the marriage separate assets they had before marriage. One of the most common assets is a residence. When the couple now married makes payments on the separate asset of one spouse from earnings the community begins to acquire an interest in the separate spouses residence.
The same rule applies when the couple makes improvements to the spouses residence and the parties use earnings or savings to make improvements on the spouses separate residence. The community interest is a complicated formula that was established from two family law cases Marriage of Moore–Marsden. The formula is know as the Moore-Maresden formula and requires the consultation of a family law attorney to understand. Below is family Code Section 2640
FAMILY.CODE SECTION 2640
When couples 2640. (a) "Contributions to the acquisition of property," as used
in this section, include downpayments, payments for improvements, and
payments that reduce the principal of a loan used to finance the
purchase or improvement of the property but do not include payments
of interest on the loan or payments made for maintenance, insurance,
or taxation of the property.
(b) In the division of the community estate under this division,
unless a party has made a written waiver of the right to
reimbursement or has signed a writing that has the effect of a
waiver, the party shall be reimbursed for the party's contributions
to the acquisition of property of the community property estate to
the extent the party traces the contributions to a separate property
source. The amount reimbursed shall be without interest or adjustment
for change in monetary values and may not exceed the net value of
the property at the time of the division.
(c) A party shall be reimbursed for the party's separate property
contributions to the acquisition of property of the other spouse's
separate property estate during the marriage, unless there has been a
transmutation in writing pursuant to Chapter 5 (commencing with
Section 850) of Part 2 of Division 4, or a written waiver of the
right to reimbursement. The amount reimbursed shall be without
interest or adjustment for change in monetary values and may not
exceed the net value of the property at the time of the division.