Side Income & Self-Employment Tax: Expenses, Home Office, Mileage & Quarterly Payments

What you'll build

What you will be able to do

Each lab ends with a concrete result that you build yourself and the checker confirms, cell by cell.

• Total a year of side-business records by category with SUMIFS, split gross income from deductible expenses, find the net profit that will be taxed, and lock the category column with a drop-down list.
• Measure the business-use share of six costs that serve both private life and the side business, apply the TTC de minimis rule with IF, and split the year's costs into a deductible and a personal part.
• Calculate the home-office deduction for three people both ways – share of the real home costs and a capped flat rate per m² – choose the higher one with MAX and translate the deduction into tax saved.
• Derive the business-use share from a driving log, depreciate the car with SLN, compare the actual-cost method with the mileage-rate method and keep the larger deduction.
• Depreciate Noor's studio equipment with SLN, count the months each asset was in service in 2026 – including an asset bought last year – and total the depreciation she can deduct.
• Calculate social contributions on three side incomes – base factor, a pension cap shared with wages, an uncapped health part – and the deductible half that lowers income for income tax.
• Compare the two safe-harbour options for Jonas, take the smaller required payment, subtract his salary withholding, split the rest into four instalments and move due dates that fall on a weekend.
• Measure how late and how short each of Jonas's instalments was, charge daily interest on late payments and on the shortfall until the return is filed, total the year and flag the late payments.