Now that your Solo 401k plan is officially established, your next step is to fund your new retirement plan.

There are two ways to fund a retirement account:

Rollovers can happen anytime during the year. There are no dollar or time limits on how many rollovers you can make into your Solo 401k plan. Rollovers are calculated separately from contributions.

This means the amount you rollover into your Solo 401k plan does not affect the amount you can contribute to your Solo 401k plan.

While rollovers can happen anytime during the year, the IRS has specific guidelines on when contributions must be made to your plan.

There are two types of contributions allowed in the Solo 401k:

Elective deferrals (employee contributions) must be formally elected by December 31st.  The actual deposit may be made  by the "due date of employer's return (including extensions)" (IRS Source)

Contribution election simply means you document that you plan to make elective deferrals into your plan and document how much you plan to contribute. 

If you complete the formal contribution election by December 31st of each year, you have until you file your tax returns for the following year to make the deposit.

Example:

From IRS Publication 560:

No, the IRS requires you to make your contribution election by December 31st for that year.

The contribution election you make by 12/31 is what stands for the year. You can change the contribution election any time before 12/31 if you are a W2 employee in your business and up until tax day if you are a sole proprietor, single-member LLC, or LLC taxed as a partnership, but you cannot make the initial election after the 12/31 deadline.

You must make your initial contribution election by 12/31. This is especially important if your Solo 401k has the Eligible Automatic Contribution Arrangement (EACA) amendment.

Yes, maybe. It depends on your business structure.

If you're a W2 wage employee in your business (e.g. because your business is an S-corp, C-Corp, or LLC taxed as an S-Corp), the IRS requires you to make your contribution election by December 31st for that year. This is the way to "lock in" your Solo 401k contribution amount.

If you are a sole proprietor, single-member LLC, or an LLC taxed as a partnership, you can change your contribution election by tax time but it is still recommended you make your initial contribution election as a best faith effort by December 31st. This is especially important if your Solo 401k has the Eligible Automatic Contribution Arrangement (EACA) amendment.

According to IRS regulations, if you are a W2 employee of your company (e.g an LLC taxed as an S-Corp, or corporation), you should have a good idea of your total annual wages before December 31st. Therefore, you should be able to elect your Solo 401k contribution by the December 31st deadline.

Suppose you're a single-member LLC or sole proprietorship. In that case, your compensation may be locked in after the 1st of the year, but you will have received any net compensation by 12/31. Therefore, you should still make your initial contribution election by the 12/31 deadline, but you can change or update your contribution election after the 1st of the New Year once you've confirmed your net compensation from the previous calendar year.

Per IRS regulations, you cannot make your deferral election after the end of the plan year on December 31st. Your employee salary deferral contribution is 3% of your total compensation.

If you did not change your contribution election by December 31st, you must contribute (at least) 3% as an employee salary deferral contribution to your Solo 401k plan.

Example: