Starting a business alongside a job is administratively easier than jumping straight into full-time self-employment. That does not mean invoices, receipts and payments can wait until the end of the year. The tax return itself rarely takes the most time; what does is working out what you invoiced to whom, when the customer paid, and which business expense has no document behind it.
This guide sets out a practical system for a side-business OSVČ (osoba samostatně výdělečně činná, the Czech term for a self-employed person) in 2026: from the first number series, through everyday record-keeping, to the paperwork for the tax return and the annual statements. It deals with business inside Czechia and with domestic documents.
What "side activity" actually means
"Side" (vedlejší) is not a type of živnost — the Czech trade licence — and not something you write on an invoice. It is above all a status for social insurance. Employment is the typical reason, but side activity can also arise while you draw an old-age or disability pension, care for a child, or study under the conditions set by law.
Saying that you only do it on the side is not enough by itself. You have to notify the reason for side activity to ČSSZ, the Czech Social Security Administration, and document it wherever the authority cannot establish it on its own. For your health insurance company, what matters is whether your employer really pays insurance for you from at least the minimum assessment base, or whether you belong to the group the minimum does not apply to.
Before you issue the first invoice: set up four things
1. Your business details
Use the same identification on your documents that you trade under: name or business name, registered address, IČO (the Czech business ID) and, if you have one, DIČ (the tax ID). Add your bank account, the payment terms and contact details. A separate business bank account is not always a legal duty for an ordinary sole trader, but in practice it makes telling private and work payments apart far easier.
2. One continuous number series
Invoice numbers have to form a clear, traceable series. 2026-0001, 2026-0002, 2026-0003 works perfectly well. Do not put the client's name into the number and do not invent a separate improvised system for each customer. When you correct an invoice, never overwrite an already-sent document so that no trace of it remains.
Taxorio assigns numbers automatically, year by year, so there is no way to use the same number twice while invoicing in the evening after work. The details are in How to set up invoice numbering.
3. A realistic due date
With a side business it is tempting to issue the invoice and stop watching it. But an unpaid invoice is still your receivable. Set a due date that matches what you agreed with the customer, and get in touch once it passes. The issue date, the date of supply and the date of payment are three different things and are not interchangeable.
4. A place for the documents you receive
Decide before your first purchase where receipts and incoming invoices will live. Photos on your phone, files in your email and paper in your backpack are not one system. Save the document right after the purchase and attach it to the expense. In Taxorio you can upload or photograph a document and let the AI read the details off it.
What a side-business invoice has to contain
How much is mandatory depends, among other things, on whether you are registered for DPH (Czech VAT) and what kind of supply you are invoicing. For an ordinary domestic invoice from a non-registered trader, make sure you have at least:
- a unique document number,
- your name, registered address and IČO,
- identification of the customer,
- a description of the service or goods supplied,
- the amount and the currency,
- the issue date and the date of supply,
- the due date and the payment method.
If you are not registered for VAT, you do not show VAT on the invoice at all. A VAT-registered trader additionally has to meet the requirements for a tax document and work with the correct rate and the correct DUZP — datum uskutečnění zdanitelného plnění, the date of taxable supply. If you are unsure about the full set of requirements, open How to issue an invoice as a sole trader.
Recording income: the invoice alone is not enough
An issued invoice says how much the customer owes you. The bank movement says when you actually got the money. For tax records you therefore need to keep issue, due date and payment apart. If a customer pays a December invoice in January, the income can fall into a different tax period than the document itself. This applies to sole traders keeping tax records (daňová evidence); if you keep full accounts, different rules apply.
A sole trader claiming expenses as a percentage of income also has to keep records of income and of receivables. Flat-rate expenses do not mean "no records". They only mean a different way of calculating the deductible expense.
| What you track | Why it matters | What to do |
|---|---|---|
| Issued invoices | They evidence the receivable and the supply you made | One number series, never delete a document you have sent |
| Payments | They decide the income received and the state of the receivable | Match them against the bank or cash regularly |
| Unpaid invoices | At year end you need a list of receivables | Go through overdue documents once a week |
| Cancellations and corrections | It has to stay traceable why an amount changed | Use the proper correction route, never rewrite history |
Recording expenses: keep the document and the business link
If you claim real expenses, a bank line reading "1,490 CZK" is not enough. You need the document, and you need to be able to explain how the spending relates to earning, securing or maintaining your business income. Typical items for a side business are software, work equipment, phone services, professional training, travel or services from other suppliers. Not every purchase that comes in handy is automatically deductible.
Where something is used both ways — a phone or a laptop you also use privately — record only the business share you could defend. For more expensive assets, depreciation rules may apply instead of a one-off expense. Flag the doubtful items and go through them with a tax adviser before you file.
Social insurance for a side-business sole trader in 2026
For social insurance, the figure that matters in 2026 is the decisive tax base of 117,521 CZK for side activity carried on throughout the year. That is not turnover and not the sum of the invoices you issued. It is the tax base from self-employment. If you did not carry on the side activity for the whole year, the amount is reduced proportionally.
Once you cross the decisive amount, participation in pension insurance becomes compulsory and the premium is calculated in your annual statement (přehled). The minimum monthly advance for side activity in 2026 is 1,574 CZK. Whether you actually pay it during the year depends on the previous year's result and on when you started.
New sole traders also get a special exemption from advances: if you did not carry on self-employment in the previous five calendar years and started no earlier than 2025, then, provided the statutory conditions are met, you do not have to pay social insurance advances in the year you start and in the immediately following calendar year (the advance is set at 0 CZK). The exemption applies to both main and side activity. This is not a waiver of the premium itself — any balance is set once you file the annual statement and is due within 8 days of filing it.
Health insurance alongside a job
If you are an employee at the same time and your employment pays insurance for you from at least the minimum assessment base, you do not have to meet the minimum assessment base as a sole trader. From 2026, such a sole trader usually does not pay monthly advances at all (neither in the first year nor in later years) and settles the premium in one sum according to the real result in the annual statement.
Check the condition with your own health insurance company, though. The situation can differ with part-time work, a low wage, several overlapping grounds, or where the employer does not pay the minimum for you and you are not covered by the state. The general minimum advance for a sole trader in 2026 is 3,306 CZK, but alongside employment it often does not apply.
A routine that takes fifteen minutes a week
- Issue the invoice as soon as the work is done. Do not count on remembering it next month.
- Upload new receipts and incoming invoices. Check the date, the amount, the supplier and the link to the business.
- Match the payments. Mark invoices as paid and look through the overdue ones.
- Check the monthly summary. Income, expenses and the estimated result show you in time whether an important threshold is approaching.
- Write down anything unclear. One note on the document beats guessing six months later what you bought.
For a deeper monthly check use Monthly closing in 15 minutes. With a side business, regularity matters more than sophistication: a short routine every week works better than one big clean-up a year.
Which thresholds to watch
The decisive amount for social insurance is not the only threshold. Also watch the turnover for compulsory VAT registration under the current rules, the conditions of the expense percentage you chose, and the limits of the flat-tax scheme (paušální daň) if it interests you. Each threshold works with a different quantity — income in one place, turnover or tax base in another. Do not mix them up.
If you are getting close to one, do not rely on adding things up in your head. Keep documents recorded as you go and deal with the situation in advance. Current amounts are listed in Minimum advances for sole traders in 2026.
What to prepare at year end
Records kept properly let you close the year without digging through old emails. Put together an overview of payments received, business expenses, unpaid receivables and liabilities, assets and advances paid. The tax return and the annual statements for ČSSZ and your health insurance company follow. Deadlines differ according to how you file and whether you are represented, so work from an up-to-date tax calendar for sole traders.
In Taxorio your issued invoices, received expenses, payment status and source documents sit in one place. You can start on the Free plan without a card — two invoices, five expenses and five AI document scans a month; if the business takes off, PRO lifts the monthly limits and adds the XML exports. The point is not to run a heavier administration. The point is to have a small system that holds up when only the evenings are left for the business.
Official sources for 2026
- ČSSZ: key figures for social security in 2026
- VZP: health insurance payments in 2026
- Financial Administration: entrepreneurs and the self-employed
Taxorio scope: Taxorio provides invoicing and income/expense records for Czech sole traders. It is not full accounting or personalised tax advice. For an unsupported or unusual case, verify the treatment with a Czech accountant or tax adviser before filing.