On your nanny's first day, two forms do the real work of turning a job offer into a legal hire: the W-4 tells you how much income tax to withhold, and the I-9 confirms your nanny can legally work in the U.S. Neither is complicated, but they have different rules about who fills out what — and when.

Form W-4 — how much tax to withhold

Your nanny completes Form W-4 so you know how much federal income tax to hold back from each paycheck. It captures their filing status and any adjustments, which feed the IRS withholding tables.

  • Your nanny fills it out — you just keep it and use the numbers.
  • It sets income-tax withholding only — not Social Security or Medicare, which are fixed percentages.
  • It can be updated anytime their situation changes (marriage, a new child, a second job).
  • Some states have their own W-4 for state income-tax withholding.
Withholding federal income tax for a household employee is technically optional — but most families do it, at the nanny's request, so they don't face a big bill in April. The W-4 is how you set it up.

Form I-9 — proof of work authorization

Form I-9 verifies your nanny is authorized to work in the United States. It has two sides, and the deadlines matter:

  • Section 1 — your nanny completes this on or before their first day.
  • Section 2 — you complete this within three business days of their start date, after physically (or via approved remote process) reviewing their identity and work-authorization documents.
  • Your nanny chooses which documents to show from the form's accepted list — you don't get to demand a specific one.
You keep the I-9 in your own records — you do NOT file it with any agency. But you must be able to produce it if asked, so store it safely for as long as the rules require.

What you do with each

  • W-4: keep it, and use it to calculate income-tax withholding each pay period. It feeds your pay stubs and, ultimately, the W-2.
  • I-9: keep it on file with a note of the documents you reviewed. Nothing gets filed, but it's your proof of a compliant hire.
Do both on day one while you're together. Chasing a signature later — especially the I-9's three-day window — is the kind of small task that quietly slips.

House walks your nanny through the W-4 and I-9 Section 1 online, and pre-fills the official forms for you to finish and sign — free.

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The bottom line

The W-4 sets income-tax withholding (nanny fills it out, you keep it); the I-9 proves work authorization (both of you complete parts, you keep it, within three days). Handle both on day one and your hire is clean from the start.