Side Business Self-Employment: How to Write Invoices Correctly Alongside Your Main Job

More and more people are discovering the benefits of side business self-employment: additional income, realizing your own ideas, and building a second career path. However, those who write invoices alongside their main job need to consider some special aspects. In this comprehensive guide, you'll learn everything you need to know about invoicing, taxes, and legal basics as a side business freelancer.

What Does Side Business Self-Employment Mean?

Side business self-employment exists when you engage in self-employed activities alongside your main occupation - usually a full-time employment. The tax office and social insurance have clear criteria:

The 20-Hour Rule

You're generally considered a side business freelancer if:

• You work no more than 20 hours per week on your self-employment
• Your income from self-employment is less than your employment salary
• Your main occupation remains your employment

Important: These limits aren't rigid rules, but guidelines. Social insurance providers evaluate each case individually.

Typical Examples of Side Business Self-Employment

• Freelancers: Web designers, copywriters, programmers, translators
• Consultants and Coaches: Business coaching, nutritional counseling, tutoring
• Creatives: Photographers, graphic designers, musicians, artists
• Craftspeople: Small weekend jobs
• Online Entrepreneurs: E-commerce, affiliate marketing, bloggers

Prerequisites: What to Clarify Before Starting

1. Check Your Employment Contract

Before you begin, review your employment contract:

Side Activity Clause: Many employment contracts contain a clause regulating side activities. This can state:

• Side activities are generally prohibited (rare and often invalid)
• Side activities must be reported
• Side activities must be approved
• No regulation (then side activity is generally permitted)

Non-Compete Agreement: You may not engage in activities that directly compete with your employer. This applies even without an explicit clause.

Practical Tip: Inform your employer in writing about your planned side activity. This creates transparency and avoids later conflicts.

2. Trade or Freelance Profession?

The distinction is important for taxes and registration requirements:

Freelance Activity (no trade registration needed):

• Doctors, lawyers, tax advisors, architects
• Journalists, writers, translators
• Artists, designers, photographers
• Lecturers, teachers, consultants
• Engineers, IT consultants (for creative work)

Commercial Activity (trade registration required):

• Trade (including online trade)
• Crafts
• Brokerage (real estate agents, sales representatives)
• Services without freelance character

Tip: If uncertain, you can request a binding ruling from the tax office or ask a tax advisor.

3. Registrations and Formalities

Freelancers:

• Complete tax registration questionnaire at tax office
• Apply for tax number
• If applicable, register with the Artists' Social Insurance (artists and publishers)

Commercial Traders:

• Trade registration at trade office (cost: approx. 20-65 €)
• Tax registration questionnaire at tax office
• IHK or HWK membership (depending on industry)

Writing Invoices as a Side Business Freelancer

Mandatory Information on Invoices

Every invoice - whether main or side business - must contain certain mandatory information (§ 14 UStG):

1. Full name and address of the service provider (your name and address)
2. Full name and address of the recipient (your customer)
3. Tax number or VAT ID (your tax number from the tax office)
4. Issue date of the invoice
5. Consecutive invoice number (unique and traceable)
6. Quantity and type of delivered goods or scope of service
7. Time of delivery or service (or period)
8. Net amount (fee without VAT)
9. Tax rate and tax amount (or reference to tax exemption)
10. Gross amount (total including VAT)

Sample Invoice for Side Business Freelancers

Max Mustermann
Musterstraße 123
12345 Musterstadt

Tax Number: 123/456/78901

                                        Musterstadt, December 23, 2025

Company Example GmbH
Beispielweg 1
54321 Beispielstadt


                    INVOICE No. 2025-042

Dear Sir or Madam,

for the services rendered, I would like to invoice as follows:

Pos.  Description                              Qty     Unit Price     Total
--------------------------------------------------------------------------------
1     Website Homepage Design                   1        800.00 €      800.00 €
      Creation of a modern, responsive
      design for your company website

2     Subpages (About, Contact, Services)       3        200.00 €      600.00 €

3     Basic SEO Optimization                    1        150.00 €      150.00 €
--------------------------------------------------------------------------------
                                                Net Amount:         1,550.00 €
                                    plus 19% VAT:                     294.50 €
                                    ====================================
                                           Invoice Total:           1,844.50 €

Service Period: December 1-20, 2025

Payable within 14 days without deduction.

Bank Details:
IBAN: DE89 3704 0044 0532 0130 00
BIC: COBADEFFXXX
Reference: RE 2025-042

Best regards
Max Mustermann

Using the Small Business Regulation

As a side business freelancer, you can benefit from the small business regulation (§ 19 UStG) if:

• Your previous year's revenue was under €22,000
• Your current year's revenue is expected to remain under €50,000

Advantages:

• No VAT on your invoices
• No VAT returns
• Less bureaucracy

Disadvantages:

• No input tax deduction on purchases
• Appears less professional to some B2B clients

Mandatory note on invoice:

"No VAT charged according to § 19 UStG."

Assigning Invoice Numbers Correctly

Your invoice numbers must be consecutive and unique. Proven systems:

Important: Gaps in numbering are allowed (e.g., for canceled invoices) but must be explainable.

Taxes for Side Business Self-Employment

Income Tax

Your profit from self-employment is added to your employment income and taxed together. This can push you into a higher tax bracket.

Sample Calculation:

Profit = Revenue - Business Expenses

Typical business expenses:

• Work equipment (laptop, software, office supplies)
• Travel costs to customers
• Training
• Proportional home office costs
• Phone and internet costs (business portion)
• Insurance (professional liability)

Sales Tax (VAT)

If you're not a small business owner, you must:

• Show 19% VAT (or 7% for certain services) on your invoices
• Submit VAT returns (monthly or quarterly)
• Remit collected VAT to tax office
• Offset input tax paid (on purchases)

VAT return deadlines:

• Monthly: By the 10th of the following month
• Quarterly: By the 10th after quarter end
• With permanent extension: Plus 1 month

Trade Tax

Trade tax only applies from a profit of €24,500. As a side business freelancer, you'll rarely reach this threshold.

Freelancers pay no trade tax - a major advantage!

Social Insurance: What You Need to Consider

Health Insurance

As a side business freelancer, you usually remain insured through your main job. But beware:

Your health insurance can classify your side activity as main occupation if:

• You work more than 20 hours per week self-employed
• Your self-employment income is higher than your employment salary
• You employ staff

Consequence of main occupation status:

• You must self-insure
• Your employer's contribution for your main job ceases
• Your employer will be informed

Practical Tip: Inform your health insurance early about the side activity. Many insurers offer consultation.

Pension Insurance

For most side business freelancers, there's no pension insurance obligation. Exceptions:

• Teachers and educators (for certain activities)
• Craftspeople (when registered in the craft register)
• Artists and publishers (Artists' Social Insurance)

Professional Association

As a commercial trader, you must register with the relevant professional association - even for side business activities. Contributions are based on your revenue.

Freelancers can insure voluntarily.

Practical Tips for Daily Life

1. Separate Business Account

Even if not mandatory: A separate account for your self-employment makes it easier:

• To track revenue and expenses
• For tax returns
• For professionalism toward customers

Many direct banks offer free business accounts for self-employed.

2. Time Management

The biggest challenge with side business self-employment is time. Tips:

• Block fixed times: For example, every evening 7-9 PM or the weekend
• Realistic deadlines: Communicate clearly that you work part-time
• Plan buffer: Main job, family, and illness can disrupt plans
• Learn to say no: Don't overcommit

3. Professionalism Despite Side Job

Even as a side business freelancer, you should be professional:

• Professional invoices: With all mandatory information, clean layout
• Fast communication: Respond promptly to inquiries
• Binding deadlines: Meet deadlines
• Clear contracts: Even for small projects

4. Proper Bookkeeping from the Start

Many side business freelancers underestimate the bookkeeping effort. Stay on top:

• Digitize receipts immediately: Use an app to photograph receipts directly
• Record revenue and expenses: At least weekly
• Build tax reserves: Approximately 30-40% of your profit for income tax
• Calendar deadlines: Tax returns, advance returns

Avoiding Common Mistakes

Mistake 1: No Tax Reserves

Many newcomers immediately spend their income and are surprised when the tax payment arrives.

Solution: Immediately set aside 30-40% of each income on a separate tax account.

Mistake 2: Not Informing Employer

If your employer learns about your side activity from other sources, it damages the trust relationship.

Solution: Proactively inform, even if no approval requirement exists.

Mistake 3: Prices Too Low

Many side business freelancers undervalue themselves because they "still have a main job."

Solution: Calculate professional prices. Your time and expertise are just as valuable as full-time freelancers.

Mistake 4: Overload

Trying to work 30 hours self-employed alongside a 40-hour employment leads to burnout.

Solution: Set realistic limits. Quality over quantity.

Mistake 5: Incomplete Invoices

Invoices without tax number or missing mandatory information appear unprofessional and can cause problems with the tax office.

Solution: Use invoicing software like Clever Invoice that automatically inserts all mandatory information.

From Side to Full-Time?

Many use side business self-employment as a springboard to full-time freelancing. When is the right time?

Indicators for the Switch

• Your side income reaches or exceeds your salary
• You have regular clients and plannable projects
• The 20-hour limit is regularly exceeded
• You want to grow but lack time
• Your main job suffers from the double burden

Checklist Before Switching

• [ ] Financial cushion for at least 6 months built up
• [ ] Health insurance clarified (statutory or private)
• [ ] Pension insurance planned (voluntary or Rürup)
• [ ] Tax advisor consulted
• [ ] Business plan created
• [ ] Customer base established
• [ ] Family and partner involved

Clever Invoice for Side Business Freelancers

With Clever Invoice you create professional invoices in seconds - perfect for those with little time alongside their main job:

How It Works

1. Enter customer data once: Name, address, payment terms - all saved
2. Create invoice via chat: "Invoice to Müller GmbH, 5 hours consulting at 80 euros"
3. All mandatory information automatically: Tax number, invoice number, date
4. PDF with one click: Professional design, ready to send
5. Stay organized: Dashboard shows open and paid invoices

Special Advantages

• Small business mode: One click, and all invoices show the §19 note
• Time-saving: Creating an invoice takes under 1 minute
• Mobile: Use on the go via smartphone
• Tax export: All data for tax return at the push of a button
• GDPR compliant: Servers in Germany, secure data storage

Conclusion: Approaching Side Business Self-Employment Correctly

Side business self-employment offers a fantastic opportunity to generate additional income and turn your passion into a career - without giving up the safety net of your main job.

The most important points summarized:

1. Check employment contract and inform employer
2. Clarify trade or freelance and register properly
3. Write professional invoices with all mandatory information
4. Use small business regulation when sensible
5. Understand taxes and build reserves
6. Coordinate social insurance with health insurance
7. Plan time realistically and set limits

With proper preparation and the right tools, nothing stands in the way of your successful side activity. Start now and gradually build your second career path!

Ready for your first invoice? With Clever Invoice you create professional invoices in seconds. Try free and start immediately!