What makes a contract term onerous?

A term that is perfectly lawful but pushes risk onto you well beyond what is normal for that kind of contract. Onerous terms are the hardest to spot because nothing about them is technically wrong. Take three examples: an uncapped indemnity, a promise to cover the other side's losses with no upper limit; a seven day time bar, a deadline so short that a genuine claim can be lost because notice was a few days late; or a set off clause reaching across other contracts, letting the other side hold back money owed on this job to cover a dispute on a different one. The law generally lets parties agree all of these, so the question is never only whether a term is legal, but whether you understand the risk it moves onto you and are being paid enough to carry it. If you cannot answer both, flag it before you sign.

Say a mechanical subcontractor takes a 90,000 pound job carrying an uncapped indemnity: a promise to cover all of the main contractor's losses if something goes wrong, with no ceiling. Nothing about the clause is unlawful, so it reads as ordinary boilerplate. Then a fault in their work contributes to a delay that ripples across the project, and the losses claimed against them climb to several times the value of their own contract. The clause was legal all along; the problem is they carried open ended risk on a fixed, modest price and were never paid to shoulder it. That mismatch, lawful wording, disproportionate risk, no extra reward, is what makes a term onerous.