A CSRS (or CSRS Offset) employee may voluntarily invest in the retirement fund up to 10 percent of the total of basic civilian salary received as of the date any contribution is made. The contributions earn interest tied to a Treasury securities rate.
If you make voluntary contributions, you may demand a refund of all (not just a part) of your account, including the interest, at any time before you retire, or at retirement. Further, at retirement you instead may purchase an additional annuity. The base amount is $7 per $100 starting at age 55; the amount increases by 20¢ for each full year you are over 55 at the time you retire. Thus, if you retire at age 60, each $100 will buy $8 a year of VC annuity, and so on. However, the additional annuity is not adjusted for inflation as are regular annuities.
A survivor benefit can be elected for this additional annuity.