Whether you need professional indemnity insurance is decided by three tests, and any one of them being yes settles it. A regulator requires it, a client contract requires it, or somebody relies on your work in a way that could cost them money if it were wrong.
Does a regulator or professional body require it
Solicitors, accountants in practice, architects, financial advisers, several healthcare professions and others must hold it as a condition of practising. Where that applies, the terms are usually specified too, and buying to a price rather than to the specification does not satisfy the obligation.
Does a client contract require it
Corporate and public sector engagements almost always name it, often alongside public liability and employers liability. Recruitment agencies place contractors on the same basis. Where this applies, the question is not whether but at what limit.
Does anybody rely on your work
If a client acts on your advice, your design, your report or your figures, and would lose money if those were wrong, the exposure exists whether anybody asks you to insure it or not. That is the honest test, and it catches a lot of small businesses who have never been asked.
Questions people ask about do i need professional indemnity insurance
Is it a legal requirement?
Not in general law. It is required by regulators for several professions and by contract for most corporate and public sector work.
I have never been asked for it. Do I need it?
If clients rely on your work and could lose money if it were wrong, the exposure exists whether or not anybody asks.
What if I only do small jobs?
The client's loss is unrelated to your fee. Small engagements can produce large losses, which is the point of the cover.