List of deductible costs/expenses from web developers, web designers, designers

Hello everyone!

I was wondering what deductible costs or expenses you include in your accounts when you design and develop websites as a self-employed person in a secondary occupation. In recent years, I have worked with an accountant. However, this accountant never indicated what costs I could incur, hence my question. Unfortunately, I did not receive much advice or guidance either.

Any tips for optimising the bookkeeping of a self-employed in secondary occupation (more specifically a web developer/web designer) are also always welcome.

Thanks in advance!

Kind regards, Vincent.