Not all modifications affect your car insurance the same way. Power-increasing modifications typically raise premiums the most, while a smaller set of safety and security upgrades can actually reduce them. What matters most to insurers is not the cost of the modification, but how it changes the car’s probability of a claim. Failing to declare any modification — regardless of cost — can void your policy.

Modifications That Increase Premiums

Insurers don’t publish a fixed percentage per modification — the impact depends on the specific insurer and how the change is documented. Roughly in order of impact, these modifications signal higher risk and will raise your premium:

Modifications That Can Reduce Premiums

A smaller number of modifications can actually reduce risk in insurers’ eyes — though the exact discount, if any, is set by each insurer individually:

• Thatcham-approved tracker (S5 or S7 category) — many insurers offer a discount for a fitted tracker, since it reduces theft risk; ask your specific insurer for the figure²
• Dashcam — some insurers offer a modest discount
• Roll cage with harness — some motorsport specialists offer reductions for properly installed HANS-compatible setups
• Thatcham Category 1 immobiliser/alarm — the gold-standard Thatcham security rating, often recognised with a discount by insurers who ask for it²

Modifications That Must Be Declared (Even If They Seem Minor)

Insurers require disclosure of any modification that differs from the manufacturer’s original specification, including:

• Tinted windows
• Sound system upgrades
• Aftermarket steering wheel
• Custom paint or vinyl wrap
• Non-standard seats
• Short-throw shifter
• Performance air filter (even a drop-in panel filter)

What Happens If You Don’t Declare?

Non-disclosure is treated as material misrepresentation. At claim time, insurers conduct a thorough inspection. If undeclared modifications are found, the insurer can void the policy from inception — meaning you receive nothing, and you may face liability for third-party costs. With specialist modified car insurers, declaring upfront almost always produces a better outcome than hiding mods and losing a claim later.

Track Use and Its Impact

Adding track use — even occasional circuit days — to your policy is a change to the policy’s permitted use. This must be declared separately from physical modifications, and priced individually rather than off a fixed percentage. Use our insurance estimator to model the combined effect of your modifications and track use on a rough budget figure — remembering it’s a planning tool, not a calibrated quote.

Sources

• Alias Insurance — Are Turbo Cars More Expensive to Insure?
• RAC — What Is a Thatcham Category Car Alarm?

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