What Do I Need to Give My CPA for My Solo 401(k)?

One of the most common questions during tax season is: What exactly does my CPA need from me for mySolo 401(k)? The good news is that a Solo 401(k) is relatively straightforward from a tax-reporting standpoint — especially compared to a full employer 401(k) plan. But getting your CPA the right information upfront means accurate deductions, full IRS compliance, and no surprises at filing time.

At MySolo401k Financial, we provide ongoing compliance support — contribution calculations, Form 1099-R and Form 5500-EZ reporting, and consultation on investment and distribution rules.

While your plan provider (like MySolo401k Financial) helps with:

• Contribution calculations
• Form 1099-R reporting
• Form 5500-EZ preparation
• Plan compliance and updates

But your CPA still needs to be in the loop on several key items. Here’s exactly what to share.

1. Contribution details & account statements

Your CPA needs to know what contributions you made to the Solo 401(k) and when. Contributions aren’t reported to MySolo401k Financial — they’re deposited directly into your Solo 401(k) holding accounts at the bank or brokerage you’ve chosen (Fidelity, Schwab, etc.).

Provide your CPA with copies of your brokerage account statements for each holding account. You’ll have separate accounts — one for each source of funds:

Pre-tax Solo 401(k) holding account — for pre-tax employee and employer contributions

Roth Solo 401(k) holding account — for Roth employee and Roth employer contributions

Voluntary after-tax holding account — for after-tax contributions (used in the Mega Backdoor Roth strategy)

The statements show when contributions were deposited, which matters because different contribution types are reported on different parts of your tax return.

Why it matters:

Different contributions are reported differently:

• Employer contributions → Business tax return (deductible)
• Employee pre-tax contributions → Personal tax return
• Roth contributions → Not deductible (but grow tax-free)

2. Your business income information

Your allowable Solo 401(k) contributions are calculated based on earned income from your self-employed business — not investment income, not passive income. Your CPA needs to confirm you had the necessary self-employment income to support your contributions.

The starting figure for the contribution calculation depends on how your business is taxed:

Once you have the right figure, you can plug it into MySolo401k Financial’s contribution calculator — or share it with us so we can run the calculation for you.

3. Where to report contributions on your tax return

Your CPA will want to know:

“Where do I report these contributions?”

This is consistently one of the top questions CPAs ask when they first encounter a client with a Solo 401(k). Here’s a quick breakdown by business type and contribution type:

Sole proprietorship / LLC taxed as sole prop

• Pre-tax employee & employer contributions → Line 16, Schedule 1 (attached to Form 1040)
• Roth employee contributions → Not reported on any tax return (no deduction, but tax-free growth)
• Roth employer profit-sharing → Deductible on Line 16, Schedule 1 and reported as a taxable in-plan conversion via Form 1099-R and on Lines 5a & 5b of Form 1040

Partnership / LLC taxed as partnership

• Pre-tax employee & employer → Line 16, Schedule 1 and Schedule K-1, Line 13, Code R
• Roth employee → Not reported on a tax return
• Roth employer profit-sharing → Line 16, Schedule 1 and K-1 Line 13 Code R, plus taxable conversion via Form 1099-R

S-Corp or C-Corp

• Employer profit-sharing (pre-tax or Roth) → S-Corp: Form 1120-S, Line 17  |  C-Corp: Form 1120, Line 23
• Employee pre-tax contributions → Schedule 1, Line 16 — or Box 12 of Form W-2 (don’t report on both)
• Roth employee contributions → Box 12a of W-2, Code AA (check Box 13 “Retirement Plan”)
• Voluntary after-tax → Not required on W-2 (optional reporting in Box 14)

4. Key Solo 401k forms to share with your CPA

Form 1099-R: Distributions & Conversions

This form reports:

• Distributions
• Roth conversions
• Rollovers

Required if you took any distributions, completed a Roth conversion (including Mega Backdoor Roth), or transferred funds out of the plan. MySolo401k Financial prepares this upon timely request. Your CPA uses it to properly report taxable events on your personal return.

Form 5500-EZ (When Applicable)

You must file Form 5500-EZ if:

• Plan assets exceed $250,000, OR
• You close the plan (regardless of balance)

Required when your plan’s market value exceeds $250,000 at year-end, or when closing the plan. Due July 31st. MySolo401k Financial prepares this when timely requested; provide your CPA a copy for their file. Even though it’s an informational filing (not a tax return), your CPA should keep it for records.

Form 8881

The Auto Contribution Credit — a $500/year credit for 3 consecutive years after plan opening. Share our video walkthrough and guide with your CPA to help them complete this two-page form.

Loan Documents

If you’ve taken a participant loan, share the loan documents and payment schedule with your CPA so they can help ensure you stay on track with repayments.

Rollover 1099-R

If you rolled over an IRA or former employer plan into the Solo 401(k), the outside institution will issue a Form 1099-R. Verify that Box 7 shows Code G (direct rollover) and share it with your CPA.

Plan Documents

If your CPA is unfamiliar with Solo 401(k)s, provide him or her with plan documents:

• Adoption Agreement
• Plan Document
• Trust Agreement
• Summary Plan Description

This helps them understand:

• Plan structure
• Contribution rules
• Available features (Roth, loans, Mega Backdoor Roth)

Share the adoption agreement, basic plan document, trust agreement, and summary plan description with your CPA so they understand the structure of your plan.

5. What your CPA does NOT need

A note on the 2026 Mandatory Roth Catch-Up rules

Starting with plan year 2026, SECURE Act 2.0 introduces mandatory Roth catch-up contributions for certain high earners. Here’s what to know:

• This does not affect 2025 contributions, regardless of when you make them.
• It only applies to businesses taxed as S-Corps or C-Corps — not sole proprietorships or partnerships.
• It applies only if your W-2 gross wages exceeded $150,000 in the prior year (2025).
• For 2026: normal catch-up is $8,000 (age 50+); super catch-up is $11,250 (ages 60–63 only).
• You cannot claim both the normal and super catch-up — only one applies.

Your CPA’s role vs. MySolo401k Financial’s role

Think of it as a triangle: your CPA handles the business and personal tax return side; MySolo401k Financial handles plan compliance, reporting, and contribution calculations; and you sit in the middle making sure both parties have the information they need.

Many of our clients are actually CPAs and tax professionals themselves — and they still rely on us for Solo 401(k)-specific guidance, because these plans have rules that differ substantially from the SEP IRAs and SIMPLE IRAs most practitioners see regularly.