Law Firm Retirement Plans: Solo 401(k), Cash Balance, and Defined Benefit Plans for Partners

A solo or small law firm has the ideal profile for aggressive retirement plan contributions: high income, few (or no) employees other than the partners, and partners who are old enough to benefit from defined benefit and cash balance plans. A solo practitioner earning $400,000 can defer over $69,000 through a solo 401(k) alone in 2025, and over $200,000 by stacking a 401(k) with a cash balance plan. The tax savings at a combined federal and state rate of 40%+ are substantial, and the money grows tax-deferred (or tax-free in a Roth 401(k)) until retirement. The catch is that if the firm has employees, the plan must cover them too, and the cost of employee contributions can offset the partners’ tax savings. Plan design for law firms is about maximizing partner contributions while minimizing the cost of covering staff.

What are the solo 401(k) limits for law firm partners?

The solo 401(k) (also called an individual 401(k) or one-participant 401(k)) is available to any self-employed individual or business owner with no full-time employees other than the owner and their spouse. For a solo practitioner or a firm where all partners are owners (not employees), the solo 401(k) is the starting point.

2025 contribution limits:

• Employee deferral: $23,500 (the amount the partner elects to defer from their own compensation)
• Age 50+ catch-up: additional $7,500 (total deferral: $31,000)
• Age 60-63 super catch-up (SECURE 2.0): additional $11,250 instead of $7,500 (total deferral: $34,750)
• Employer profit-sharing: up to 25% of net self-employment income (for sole proprietors and partnerships) or 25% of W-2 compensation (for S-corp shareholder-employees)
• Combined maximum (employee + employer): $70,000 ($77,500 for 50+, $81,250 for 60-63)

For a partner in a law firm organized as a partnership (or LLC taxed as a partnership), the employer contribution is based on “net self-employment income,” which is the partner’s net earnings from self-employment reduced by one-half of self-employment tax and by the partner’s own employee deferral. The calculation is circular (the deferral reduces the base, which reduces the maximum employer contribution), but IRS Publication 560 provides worksheets and the formula resolves to approximately 20% of net earnings for the employer portion.

Roth option. The solo 401(k) can include a Roth component: the employee deferral ($23,500 or more with catch-up) can be designated as Roth (after-tax contribution, tax-free growth and withdrawal). The employer profit-sharing contribution is always pre-tax. SECURE 2.0 allows employer contributions to be designated as Roth starting in 2024, but the practical implementation is still evolving.

Deadline. The plan must be established by December 31 of the tax year (for the plan to exist). Employee deferrals must be elected by December 31. Employer profit-sharing contributions can be made up to the tax return filing deadline (including extensions), so a partner can wait until the return is prepared to determine the optimal contribution.

When does a cash balance plan make sense?

A cash balance plan is a type of defined benefit plan that defines the benefit as a hypothetical account balance (a “cash balance”) rather than a monthly annuity. The employer makes contributions that are actuarially determined to fund the promised benefit, and the contributions are deductible under IRC 404.

The contribution limits for a cash balance plan are much higher than for a 401(k) because they are based on the actuarial funding needed to provide the promised benefit at retirement age, and older participants need larger annual contributions to fund the same benefit in fewer years.

Approximate annual contribution limits by age (2025, illustrative):

• Age 35: $60,000-$80,000
• Age 40: $80,000-$110,000
• Age 45: $110,000-$150,000
• Age 50: $150,000-$200,000
• Age 55: $200,000-$280,000
• Age 60: $280,000-$350,000+

These are approximate because the actual contribution is determined by the plan’s actuary based on the interest crediting rate, the assumed retirement age, and the participant’s compensation history. The key point: a 55-year-old partner can contribute $200,000-$280,000 to a cash balance plan in addition to the $77,500 solo 401(k), for a combined contribution exceeding $280,000 in a single year.

When it makes sense:

• The partner(s) are age 45+ (the contribution limits increase significantly with age)
• The firm’s income is consistently high ($300,000+ per partner) and expected to remain high for at least 5-7 years (the plan must be maintained for a reasonable period)
• The firm has few employees (the cash balance plan must cover eligible employees, and the employer contributions for employees add cost)
• The partner’s marginal tax rate is 35%+ (the tax savings from a $200,000 contribution at 37% marginal rate is $74,000)

When it does not make sense:

• The firm’s income is volatile (the contribution is mandatory once the plan is established; a down year with a large mandatory contribution creates a cash crunch)
• The firm has a large staff relative to the partners (the required employee contributions can exceed the partners’ tax savings)
• The partners are under 40 (the contribution limits are not dramatically higher than a 401(k) profit-sharing contribution)

How do you stack multiple plans?

The most common stacking strategy for law firms is 401(k) + cash balance plan:

Layer 1: 401(k) employee deferral. The partner defers $23,500-$34,750 (depending on age) into the 401(k). This is the easiest and most flexible layer because the deferral amount can change each year.

Layer 2: 401(k) employer profit-sharing. The firm contributes up to 25% of net self-employment income (or W-2 for S-corp) as a profit-sharing contribution. Combined with the deferral, this can reach $70,000-$81,250.

Layer 3: Cash balance defined benefit plan. The firm establishes a cash balance plan on top of the 401(k). The actuary determines the annual contribution needed to fund the promised benefit. The combined deduction for the 401(k) and cash balance plan is limited to 25% of total covered compensation for the 401(k) employer contribution, but the cash balance plan has its own deduction limit under IRC 404(a)(1) that is separate from the 401(k) limit.

Total annual tax-deferred contribution (example, age 55 partner earning $500,000):

• 401(k) deferral: $31,000 (age 50+ catch-up)
• 401(k) profit-sharing: ~$39,000 (to reach the $70,000 combined limit, adjusted for self-employment tax)
• Cash balance plan: ~$200,000 (actuarially determined)
• Total: ~$270,000 in tax-deferred contributions
• Tax savings at 37% federal + 5% state: ~$113,000 in reduced current-year tax

What are the employee coverage requirements?

If the firm has employees (associates, paralegals, secretaries, receptionists), the retirement plan must satisfy coverage and nondiscrimination rules under IRC 401(a)(4) and 410(b). The rules are designed to prevent plans that benefit only the owners:

Eligibility. Employees who have completed one year of service (1,000+ hours) and are age 21+ must generally be eligible to participate. The firm can exclude employees with less than one year of service.

401(k) safe harbor. The easiest way to satisfy the nondiscrimination rules for the 401(k) is the safe harbor contribution: the firm makes a 3% non-elective contribution to all eligible employees (regardless of whether the employee defers), or a 4% match on employee deferrals. The cost of the safe harbor contribution is the price of allowing the partners to defer the maximum.

Cash balance plan coverage. The cash balance plan must also cover eligible employees, and the benefit formula for employees must not discriminate in favor of highly compensated employees. In practice, the actuary designs the plan so that the partners receive the maximum benefit and the employees receive a smaller (but compliant) benefit. The employee cost is typically 5-7% of covered employee compensation, which the firm must fund each year.

Cross-testing. Some plans use “cross-testing” (also called “new comparability”) to demonstrate nondiscrimination. Cross-testing converts the defined contribution and defined benefit amounts to equivalent benefits and tests whether the benefit rates are nondiscriminatory when compared on a benefits basis rather than a contributions basis. This technique can reduce the required employee contributions while maintaining the high partner contributions, but it requires careful actuarial design.

How does the SSTB classification affect the strategy?

Law firms are classified as Specified Service Trades or Businesses (SSTBs) under IRC 199A(d)(2), which means partners with taxable income above the threshold ($191,950 single, $383,900 MFJ for 2025) receive no qualified business income (QBI) deduction. The 20% QBI deduction is not available.

This makes retirement plan contributions more valuable for law firm partners than for non-SSTB business owners. A non-SSTB business owner earning $500,000 gets both the QBI deduction (up to $100,000 x 20% = $20,000 deduction, subject to W-2/UBIA limits) and retirement plan contributions. A law firm partner earning the same amount gets only the retirement plan deduction.

The retirement plan is the primary tool for reducing the law firm partner’s taxable income below the top bracket. Without it, the partner pays federal tax at 37% on income above $609,350 (2025 MFJ), plus state income tax, plus the 3.8% net investment income tax on investment income (and the 0.9% additional Medicare tax on SE income above $250,000 MFJ). A $200,000 cash balance contribution reduces the effective tax rate meaningfully.

What should I do next?

If your firm does not have a retirement plan, start with the solo 401(k) (if no employees) or a safe harbor 401(k) (if employees exist). If the partners are 45+ and earning $300,000+, model the cash balance plan: the actuarial feasibility study typically costs $2,000-$5,000 and tells you the contribution range and employee cost before you commit. If you already have a 401(k) and are maxing the profit-sharing contribution, the cash balance plan is the next layer.

• Law firm partner compensation, how guaranteed payments, distributive shares, and SE tax interact with retirement plan contributions
• Law firm tax deductions, the other deductions that reduce taxable income alongside retirement plan contributions
• Law firm bookkeeping, the chart of accounts and monthly close that produces the income data for retirement plan calculations
• Law firm billing and collections, the revenue that funds the retirement plan contributions
• Restaurant entity structure, the parallel S-corp election and retirement plan strategy for another high-income professional service business

This guide is general information, not tax advice for your specific situation. Which points apply, and how, depends on your facts.