There is no single figure, and the "from" prices published elsewhere are drawn from the lowest-risk business an insurer will accept. Public liability is rated from a base rate for your trade, applied to a measure of your exposure — usually turnover — and then adjusted for everything that makes your business different from the average business in that trade. This guide sets out what an underwriter actually looks at, so that when you receive a quote you understand what produced it.
An underwriter is not pricing a product, they are pricing a probability: how likely is it that this business injures someone or damages their property, how expensive would that be, and what would it cost to defend the claim even if the business did nothing wrong. In practice that means a base rate for the trade, applied to turnover or wageroll, then adjusted. The insurer's own appetite sits on top — one actively writing your sector will rate it more keenly than one accepting it reluctantly.
Trade and the actual activities carried out matter more than the trade label, because two businesses filed under the same heading can carry very different risk. Beyond that: turnover, whether you employ staff or use subcontractors, where you work and whether that includes clients' premises or work at height, the materials and processes involved, your claims history, the indemnity limit you need, and the excess you carry.
The indemnity limit is the maximum an insurer will pay for any one claim, and policies are typically written at one of four levels. Moving up a level does not move the premium proportionally, because the great majority of claims settle well below even the lowest limit — the additional cost buys protection against the rare severe claim. Many contracts and public-sector frameworks specify a minimum limit, so the requirement is often set for you. The excess is what you pay towards each claim, and increasing it voluntarily reduces the premium at the cost of carrying more of each loss yourself.
Usually because they are not as similar as they look: different declared activities, a different turnover band, a claim on one record and not the other, a different indemnity limit, or simply two insurers with different appetite for the sector. Sole traders ask about cost more than any other group, and the picture changes for them mainly through the absence of employers' liability and a lower exposure measure, not through a different kind of policy.
A quote is only as good as the information behind it. Have your actual activities described in your own words rather than a trade category, your projected turnover, your claims history over the last several years, the indemnity limit any contract requires, and details of subcontractors you use. A comparison form reduces a business to a set of dropdowns, and picking the nearest option when your trade is not listed is how cover ends up not matching the work. Premiums also move at renewal when nothing about the business has changed, because insurers rate forward for the expected future cost of claims across the whole book.
Cover is subject to underwriting, insurer terms and acceptance