What does a TPA actually do — and when do you need one?

Ask a business owner who their 401(k) is "with" and they'll name a recordkeeper — the company whose logo is on the website. Ask who keeps that plan out of trouble with the IRS and the Department of Labor, and you'll usually get a blank look. That second job belongs to the plan's administrator, and when it's done by an independent firm, that firm is a third-party administrator — a TPA.

The three-legged stool, briefly

Every 401(k) involves three functions: recordkeeping (the ledger — balances, contributions, statements, the app), administration (the legal machinery — documents, testing, filings), and investments/advice. In a bundled plan, one vendor does everything. In an unbundled plan, an independent TPA handles administration alongside a recordkeeper.

This article is about that middle leg, because it's the one nobody sees until it fails.

What the job actually consists of

The plan document. A 401(k) exists as a legal document — eligibility, match formulas, vesting, distributions, every feature your plan does or doesn't have. TPAs draft it, amend it as your business changes, and restate it on the IRS's required cycle [VERIFY: current restatement cycle]. When an employee asks "am I eligible?" or a divorce attorney sends a QDRO, the answer comes out of this document.

Nondiscrimination testing. Congress grants 401(k) tax benefits on the condition that plans don't exist purely to benefit owners and executives. The enforcement mechanism is an annual battery of tests — ADP/ACP (comparing deferral and match rates of highly compensated employees against everyone else), top-heavy testing, coverage testing, and the 415 limits [VERIFY: test names and mechanics]. Fail the ADP test and money literally comes back out of the owners' accounts as taxable refunds. A capable TPA doesn't just run these tests in February — they project them in October, while there's still time to fix the outcome.

Form 5500 and the government-facing calendar. The annual Form 5500 filing, the audit coordination once your participant count crosses the threshold [VERIFY: current 100-participant / participants-with-balances rule], required participant notices on required schedules, and the deadlines nobody tells you about until they're missed.

Fixing what breaks. Plans break constantly, in ordinary ways: a new hire never got enrolled, deferrals were deposited late, the match was calculated on the wrong definition of compensation, somebody exceeded a limit. The IRS runs a formal correction program for exactly this [VERIFY: current EPCRS/self-correction scope], and navigating it is bread-and-butter TPA work. The difference between a $500 correction and a five-figure one is usually how early a competent person got involved.

Plan design — the part that makes owners money. This is the job that separates real TPAs from filing services. Given your census — ages, salaries, ownership — a good TPA engineers the plan: safe harbor to bypass testing entirely, new comparability profit sharing that directs a legally maximized share of employer dollars to owners, eligibility timing that fits your workforce, and add-ons like cash balance plans when owner savings goals outgrow the 401(k)'s limits [VERIFY: current combined-plan limits]. I've seen redesigns move an owner from $30,000 to $70,000+ of annual tax-advantaged savings with a modest, predictable increase in staff cost. That's not a loophole; it's what the rules allow when someone actually reads them.

TPA vs bundled: when "included" administration is enough

Bundled providers do perform administration, and for a lot of plans it's genuinely fine. The honest comparison looks like this:

The pattern I'd summarize after years around both models: bundled administration is built to be scalable, and scale means the service degrades exactly when your situation stops being standard.

Signals you've outgrown "included"

• You're an owner who wants to contribute meaningfully more than the deferral limit, and nobody has ever shown you a profit-sharing illustration.
• Your workforce is lopsided — a few older, well-paid owners and a younger staff — which is precisely the setup where custom design pays.
• You've had testing failures or refunds, or top-heavy status arrived as a surprise.
• You're acquiring a business (their plan's problems become your problems — plan issues belong on every due-diligence checklist).
• You received any correspondence from the IRS or DOL about the plan.
• Your CPA asks a plan question and the provider's answer starts with "you should consult your own advisors."

None of these mean your recordkeeper is bad. They mean the administration leg of your plan now deserves a specialist whose only job is your compliance — and whose fee, typically a few thousand dollars a year [VERIFY: typical small-plan TPA pricing], is small against either the design upside or the correction downside.

How to pick one

Ask a prospective TPA the same style of questions you'd ask any provider: Who signs the 5500 — do you offer 3(16) services and which duties, specifically, do you take on? Will you run a design study from our actual census before we sign? How do you price corrections? Do you take revenue sharing from any recordkeeper, or are your fees fully disclosed and independent? A good TPA answers with specifics and a sample illustration. A filing mill answers with a rate sheet.

The retirement-plan industry spends its marketing budget on apps and fund lineups. The unglamorous truth is that for a small business, the person who most determines whether your plan quietly builds wealth or quietly builds liability is the administrator. Choose that one on purpose.