Simplified tax advances for 2027: the base is the 2025 return, so the client list belongs in November

A 2027 Payment Set by 2025 Income

Simplified advances look like the least interesting decision of the year. A client on the tax scale or the flat 19%, or a company on CIT, pays the same amount every month: 1/12 of the tax shown in the return filed in the previous year. No monthly calculation of the advance, no surprises in the bank statement. Clients like it, and plenty of offices like it too.

The catch sits in the calendar. For 2027 the base is the return filed in 2026, which is the return for 2025. So every simplified advance a client pays in 2027 is set by what they earned two years earlier. If the 2025 return showed no tax, the base moves back one more year, to the return for 2024. If that shows no tax either, the method is closed for 2027.

Two years is a long time for a small business. A client who lost their biggest customer in 2026 will pay 2027 advances as if it were still 2025. A client who doubled their revenue will pay advances as if they hadn't. Neither is breaking the law. Both deserve a quick word in November, before the first 2027 advance is paid.

Two Clients, Same Method, Opposite Outcomes

Two sole traders on the flat 19%. To keep it readable I'll compare tax only and leave the health contribution out. It moves the amounts, not the direction.

The shrinking client. 2025 income: 300 000 zł, so the 2025 return shows 57 000 zł of tax. The simplified advance for 2027 is 4 750 zł a month. But a major contract ended this year, and 2027 income is projected at 180 000 zł, which is 34 200 zł of tax. Over twelve months the client pays 57 000 zł in advances against 34 200 zł of actual tax. 22 800 zł of overpayment sits with the tax office until the 2027 return is filed in spring 2028 and the refund comes back. For a business that just lost 40% of its income, that's a loan to the state at 0%, made in the worst possible year.

The growing client. 2025 income: 120 000 zł, 22 800 zł of tax, so the simplified advance is 1 900 zł a month. 2027 income is projected at 260 000 zł, 49 400 zł of tax. During 2027 the client pays 22 800 zł and owes a 26 600 zł top-up with the annual return, by 30 April 2028. That's a real cash-flow advantage, provided the client knows it's coming and keeps the money. If nobody tells them, April 2028 is a bad month.

Same method, same year, opposite advice. The difference is entirely in one ratio: expected 2027 income against 2025 income.

Who Can't Use It at All

Before the ratio, the eligibility check:

• Lump-sum clients. In PIT the method exists only for the tax scale and the flat tax. A client switching to the lump sum for 2027 (the form-of-taxation conversation happens in the same weeks) drops out of this list automatically.
• Clients who started in 2026 or 2027. The method is closed to anyone who started their business in the tax year or the year before it.
• No tax in either base return. Both the 2025 and the 2024 return showed zero tax: no simplified advances in 2027.
• A corrected base return. If the 2025 return was corrected, the advance may change, and depending on when the correction was filed, the right to the method can be affected. CIT has an explicit rule for this. Whatever the form of taxation, a correction filed in 2026 should be a flag on the list, not something the office remembers in March.

For companies the logic is the same. 1/12 of the tax due from the CIT-8 filed in the previous year, so for 2027 the CIT-8 for 2025, with the same one-year fallback and the same start-up exclusion. A small taxpayer has a third option, quarterly advances, which some offices quietly prefer for small companies anyway.

There's No Form, Which Is the Problem

Nothing is filed in January. In practice the choice shows in the first advance paid at the simplified amount, and the information that the client used the method goes into the annual return for 2027: PIT-36 or PIT-36L, or CIT-8, filed in 2028. Once chosen, it applies for the whole year. There's no switching back in June when the numbers turn out differently.

So the decision is made by default. A client who paid simplified advances in 2026 keeps paying them in 2027, at a new amount from a newer return, and nobody stops to ask whether the new amount still makes sense. A client who never used the method never hears about it, even when it would have deferred twenty thousand złoty.

How Many Clients This Is

Take a typical office I work with: around 120 clients, 60 on KPiR, 25 on full books, 35 on the lump sum. The question only applies to the 85 outside the lump sum, minus anyone who started in the last two years. In my experience, 10 to 15 of them will have income in 2026 that differs from 2025 by more than 30%, up or down. Those are the clients where the method either helps a lot or hurts a lot.

Finding them by hand means opening two things per client: the 2025 return in the archive, and the 2026 ledger in the accounting program. Ten minutes each, if the archive is tidy. For 85 clients, two days that nobody has in November. So the 2027 advances get paid the way the 2026 ones were.

The mistakes repeat:

• The new amount is never compared with the new reality. The office recalculates 1/12 from the 2025 return correctly and stops there.
• The growing client isn't warned about April. The deferral is an advantage only if the top-up is planned.
• The fallback year is forgotten. A client with a loss in 2025 but tax in 2024 can still use the method, at an amount set by 2024 income. That's three years of lag.
• A corrected 2025 return goes unnoticed because the correction was filed by someone else, or months later.

What the Agent Prepares

An agent with read access to the office's return archive and the client ledgers can assemble the list in November, as part of the month-end close the office already runs:

• Reads the tax due from each client's 2025 return (and the 2024 one where 2025 shows no tax), plus any correction filed since.
• Reads 2026 income from January to the last closed month and takes the office's 2027 projection, or projects from the last twelve months rather than multiplying ten months by 1.2.
• Checks eligibility: current and planned form of taxation, business start date, whether the base return was corrected.
• Computes the simplified advance for 2027 and compares the year's total with the expected 2027 tax. It flags clients where the gap exceeds a threshold the office sets, say 5 000 zł, in either direction.
• Writes one line per flagged client: "overpays about 22 800 zł in 2027, consider regular advances" or "defers about 26 600 zł to April 2028, client needs to plan for it."

The agent doesn't choose the method and doesn't pay anything. The accountant decides with the client, ideally in the same November conversation as the form of taxation, because the two answers affect each other.

One Thing to Do This Week

Take your clients who paid simplified advances in 2026. Next to each name write two numbers: income in the 2025 return, and income booked for 2026 so far, annualised. Any client where the second number is less than 70% of the first should hear from you before the first 2027 advance is paid.

If you'd like to see this list built from your own clients' returns and ledgers, write to me. I'll take one month of real data and show you which clients the agent flags and why.