Any change to your car that alters it from the manufacturer's original factory specification counts as a modification in the eyes of insurers, and it must be declared. Failing to disclose modifications can invalidate your policy entirely, meaning your insurer could refuse to pay out on claims. This guide explains what counts, how different modifications affect your premium, and how to stay properly covered.
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A modification is any change made to your car that alters it from the manufacturer's original factory specification. This definition is deliberately broad and covers far more than most drivers realise. It includes performance changes, cosmetic changes, security additions, and even practical accessories.
| Category | Examples | Typical effect on premium |
|---|---|---|
| Performance | Engine remaps (ECU tuning), turbochargers, superchargers, sports exhausts, air filter upgrades, uprated brakes, suspension changes | Significant increase - performance gains raise accident risk in insurers' models |
| Cosmetic | Alloy wheels, body kits, spoilers, custom paint, vinyl wraps, tinted windows, interior upgrades, custom lighting | Small to moderate increase - higher repair costs and theft attraction |
| Security | Thatcham-approved alarms, immobilisers, GPS trackers, steering wheel locks | Can reduce your premium - lowers theft risk |
| Practical | Tow bars, roof racks, parking sensors, dash cams | Minimal impact - some may slightly reduce your premium |
Even selecting alloy wheels or upgraded trim as an optional extra when buying a new car is technically a modification that should be declared. If you have bought a car that was previously modified by someone else, those changes still need to be disclosed on your policy.
Most modifications increase your premium because they change the risk profile of your car. The size of the increase depends on what you have changed and how it affects the insurer's assessment of repair costs, performance, vehicle value, and theft risk.
Modified parts are typically more expensive to fix or replace than standard components. Custom bodywork, performance brakes, or upgraded exhausts require specialist labour and parts that are not always readily available. With repair costs making up the largest share of motor insurance claims (ABI, Q1 2026), insurers are particularly sensitive to anything that increases potential repair bills.
Modifications often increase your car's overall value. If your vehicle is written off or stolen, the insurer faces a larger payout. This is particularly relevant for comprehensive policies where the insurer covers the full value of the vehicle.
Engine remaps, turbochargers, or exhaust upgrades can significantly increase your car's power output. Insurers view this as heightened accident risk, particularly for younger or less experienced drivers. Even handling modifications like lowered suspension can affect how your car behaves in emergency situations. For more on how your car choice affects insurance, see our guide on car insurance groups explained.
Modified cars can be more attractive to thieves, especially those with expensive visible upgrades or eye-catching custom features. The ABI reports that the average theft claim runs into thousands of pounds, so insurers factor theft risk heavily into their calculations for modified vehicles.
Not all modifications push your premium up. Security-focused changes can actually bring costs down, and insurers actively reward drivers who take steps to protect their vehicles.
Thatcham Research is the automotive industry's security testing body, and insurers place significant value on their certification. Installing Thatcham-approved security can lead to meaningful premium reductions.
GPS trackers (Category S5 or S7) significantly increase the chance of recovering a stolen vehicle. Many insurers offer discounts for fitted trackers, and some require them for high-value or high-risk vehicles. Upgraded alarms and immobilisers beyond the factory standard can also help, particularly in high-crime postcodes.
Parking sensors reduce the likelihood of minor bumps and scrapes, which can positively affect your premium over time. Dash cams may not always attract a direct discount, but the footage can help prove you were not at fault in an accident, protecting your no claims bonus. Winter tyres should not negatively impact your premium either - over 70 insurance companies have signed agreements not to increase premiums for declaring them.
If you are considering modifications, check the insurance implications before you start. Getting a quote with and without the planned modification tells you exactly what it will cost. Compare car insurance via Brumble to see how different insurers price your modifications.
You must tell your insurance company about any changes to your vehicle, no matter how minor they seem. The Financial Ombudsman Service has highlighted increasing numbers of cases where drivers have faced significant financial losses because they did not declare modifications.
If you do not declare modifications and need to make a claim, your insurer could refuse to pay out entirely, void your policy as if it never existed, charge you the full difference in premium that should have been paid, or cancel your policy, making it harder and more expensive to get insurance in future.
This applies whether you made the modifications yourself or bought a car that was already modified. In one Financial Ombudsman case, a driver had their claim rejected because the car had alloy wheels larger than standard, even though the driver said they did not know about the modification. The responsibility to check and declare sits with the policyholder.
If you are unsure whether something counts as a modification, the safest approach is to declare it. Your insurer would rather know about a minor change upfront than discover it undeclared at the point of a claim. For more on how to keep your policy valid, see our guide on how to lower your car insurance premium without cutting corners.
Some modifications are not just expensive to insure - they are illegal. Fitting any of these could result in your car failing its MOT, being seized by police, or earning you a fine.
| Modification | Legal limit |
|---|---|
| Window tinting (front windscreen) | Must allow at least 75% of light through |
| Window tinting (front side windows) | Must allow at least 70% of light through |
| Exhaust noise | Cannot exceed 74 decibels (dropping to 68dB for new cars from 2026) |
| Lighting | Blue and green lights are prohibited, as are flashing LED underlights |
| Catalytic converter removal | Tampering with emissions control systems is illegal |
| Unsafe tyre configurations | Stretched tyres and mixed tyre types can be illegal and will fail an MOT |
Different insurers treat modifications very differently. Some mainstream insurers load premiums heavily for any modifications, while specialist providers may offer significantly better rates for the same vehicle. This is why comparing quotes is essential if your car is modified.
When getting quotes, be completely honest and detailed about all modifications. Provide specific information about brands, specifications, and whether work was done professionally. This helps insurers give you accurate quotes and ensures you are fully covered if you need to claim.
Building your no claims bonus remains one of the most effective ways to reduce premiums whether your car is modified or not. After five or more years of claim-free driving, it can cut your costs by up to 60%. For young drivers with modified cars, a black box policy can also help demonstrate safe driving and bring costs down.
Most modifications increase your premium because they change the risk profile of your car. Performance modifications like engine remaps and turbochargers have the biggest impact, while cosmetic changes like alloy wheels or body kits cause smaller increases. The exception is security modifications like Thatcham-approved trackers and alarms, which can reduce your premium.
Yes. Any change from the manufacturer's standard factory specification counts as a modification, including optional extras selected when buying a new car. Upgraded alloy wheels, larger engines, sport suspension, and premium sound systems should all be declared to your insurer.
You must declare all modifications, no matter how minor. This includes performance changes (engine remaps, exhausts, turbochargers), cosmetic changes (alloy wheels, body kits, tinted windows, wraps), security additions (alarms, trackers), and practical accessories (tow bars, roof racks). If in doubt, declare it - your insurer would rather know upfront than discover it at the point of a claim.
Yes, a vinyl wrap is a modification and must be declared to your insurer. It changes the vehicle's appearance from its factory specification, which can affect its value, repair costs, and theft risk. The impact on your premium is typically small, but failing to declare a wrap could invalidate your policy if you need to claim.
Yes. Non-standard alloy wheels must be declared and can cause a small to moderate increase in your premium. Larger or more expensive wheels increase the cost of any repair or replacement claim. In one Financial Ombudsman case, a driver had their claim rejected because the car had alloy wheels larger than standard.
Some modifications can void your manufacturer warranty, particularly performance changes like engine remaps or turbocharger installations that put additional stress on the drivetrain. Cosmetic changes are less likely to affect your warranty. Check with your dealer before making any changes if your car is still under warranty.
Most modifications are legal as long as they comply with MOT standards and road traffic regulations. Illegal modifications include excessive window tinting (below 75% light on windscreen or 70% on front side windows), exhausts exceeding 74 decibels, blue or green lights, removed catalytic converters, and unsafe tyre configurations. Always check that any planned modification is road-legal before fitting it.
The best way to get competitive modified car insurance is to compare quotes from multiple providers, including specialists who understand modified vehicles. Different insurers price modifications very differently, so the cheapest mainstream insurer for a standard car may not be the cheapest for a modified one. Compare car insurance via Brumble to search across 130+ insurers.
Association of British Insurers - Motor Insurance Premium Tracker and Claims Data, Q1 2026
Financial Ombudsman Service - Car modifications and insurance complaints guidance
Thatcham Research - Vehicle security device certification and insurance group methodology
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