Before you start

An SSN or ITIN
Form W-4 with your employer for federal withholding
California Form DE 4 with your employer for state withholding
Records of foreign income and foreign financial accounts
The Mello-Roos disclosure for any property you are buying

Step-by-step

1
Complete the federal W-4 and California DE 4 on day one
California's withholding certificate is separate from the federal one, and using the federal allowances for both is a common source of under-withholding. Fill in the DE 4 properly.
Via employerWho: YouFirst week of employment
2
Determine your US tax residency
The substantial presence test counts weighted days across three years to decide whether the US taxes your worldwide income or only US-source income. Your visa category does not decide this.
OnlineWho: You
3
Determine your CALIFORNIA residency separately
California applies its own residency analysis — a facts-and-circumstances test centred on domicile and closest connections — and administers it rigorously. Someone can be a California resident for state purposes on a pattern that would not make them a resident elsewhere.
OnlineWho: You
4
Check for an applicable tax treaty, and check whether California honours it
The US has treaties with around 70 countries. California does not automatically conform to federal treaty provisions — a position exempt from federal tax under a treaty can still be taxable by California. This is a genuine trap for newcomers who model the federal answer and assume the state follows.
OnlineWho: You
5
Remember California taxes capital gains as ordinary income
There is no preferential state rate for long-term capital gains — they are taxed at your full marginal California rate, on top of the federal treatment.
OnlineWho: You
6
If you buy, check the Mello-Roos before anything else
Proposition 13 pegs your taxable value to what you paid, rising about 2% a year until you sell, so budget from the purchase price rather than from the neighbour's bill. Then check whether the parcel sits in a Mello-Roos community facilities district, which adds a separate special tax that Proposition 13 does not limit. In Riverside County's newer subdivisions this is the rule rather than the exception, it can add substantially to the annual bill, and it is disclosed at sale precisely because people miss it.
OnlineWho: You
7
Budget the 8.75% sales tax, and notice it is not the county rate
California's statewide base is 7.25% and the City of Riverside sits at 8.750%, above the 7.750% that applies across much of unincorporated Riverside County and above several neighbouring cities. Groceries and prescription medicine are exempt; prepared food is not. On a household's annual taxable spending the difference between 7.75% and 8.75% is real money.
OnlineWho: You
8
File federal and California returns by 15 April
The tax year is the calendar year. California's filing deadline normally aligns with the federal one, and the state has extended it in disaster years — check rather than assuming either way.
OnlineWho: YouBy 15 April annually
9
File an FBAR if foreign accounts exceed $10,000
Aggregate foreign account balances over $10,000 at any point in the year trigger a FinCEN filing, separate from your tax return and with its own penalties.
OnlineWho: You

Documents you’ll need

Form W-2 from each employer, issued by 31 January
Form 1099s for freelance, interest and investment income
Forms W-4 and DE 4 filed with your employer
Mello-Roos and special-assessment disclosures, if you are buying
Passport and travel history for the substantial presence day count
Foreign account statements for FBAR reporting

Things most newcomers don’t know

Mello-Roos is the Riverside County number people miss, and Proposition 13 does not restrain it.

Proposition 13 caps the growth of your assessed value; a Mello-Roos community facilities district special tax is a separate levy that funds the infrastructure of the subdivision and sits outside that cap. Much of the Inland Empire's post-1990 housing stock is inside one. Two identical houses a mile apart can differ by thousands of dollars a year on this line alone, and it is invisible in an asking price. Read the disclosure before the offer, not after.

Source: Riverside County Assessor-County Clerk-Recorder

Riverside's sales tax is a full point above much of its own county.

The City of Riverside sits at 8.750% while much of Riverside County is at 7.750%. Anaheim, across the county line, is also 7.750%. It is a small percentage and a real annual sum on a household's taxable spending, and it is one of the few costs where moving inland does not save you money.

Source: California Department of Tax and Fee Administration

The SDI wage ceiling was removed in 2024, and it applies to every dollar.

State Disability Insurance used to stop at a wage cap. Since 2024 it applies to all wages, which functions as an additional percentage point on top of an already high marginal rate for higher earners. It is deducted quietly on the payslip and salary comparisons between states routinely miss it.

Source: California Employment Development Department

California does not automatically follow federal tax treaties.

A newcomer whose income is exempt from federal tax under a treaty with their home country can still owe California tax on it, because the state does not conform to those provisions. This surprises people every year, and it is the single most valuable thing to check with a professional in your first California filing season.

Source: California Franchise Tax Board

Common mistakes to avoid

Buying in a Mello-Roos district without pricing the special tax — in this county, assume there is one until shown otherwise.
Filling in the federal W-4 and ignoring the separate California DE 4.
Assuming a federal treaty exemption also applies to California. It frequently does not.
Budgeting property tax from a neighbour's bill rather than from your purchase price.
Assuming the county sales tax rate applies inside the City of Riverside. It does not.
Missing the FBAR because a home-country account did not feel 'foreign'.

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.