Compulsory and voluntary excess add together
Your total excess is the compulsory excess plus the voluntary excess — these add together, they don't replace each other. This is one of the most common points of confusion when making a claim: someone who chose a £250 voluntary excess to lower their premium is often surprised to also owe the separate compulsory excess set by the insurer on top of it. For example, a £250 voluntary excess plus a £300 compulsory excess means you pay £550 in total before your insurer covers the rest — not £250 or £300 alone. If a claim is smaller than your total excess, you pay the whole claim yourself and the insurer pays nothing.
Some policies also add other excesses in particular situations — for example for younger or less experienced drivers, or for certain types of claim. If one applies to your claim, check your policy schedule and include it in the compulsory figure.
Raising your voluntary excess can lower your premium, but it only makes financial sense if you're confident you could actually afford the higher total excess if you ever need to claim. This calculator shows the claim-time cost, not whether a given trade-off is right for your budget.