What counts as a modification?
A modification is any change from the manufacturer's factory specification — and the definition is far broader than most drivers realise. If the car didn't leave the factory that way, insurers treat it as a modification, whether it was fitted by you, by a dealer, or by a previous owner. It falls into a few groups:
- Performance — engine remaps or chips, turbo or supercharger upgrades, exhaust systems, air intake or induction kits.
- Handling — lowered or uprated suspension, coilovers, anti-roll bars, bigger brakes.
- Cosmetic — alloy wheels, body kits, spoilers, custom paint or wraps, window tints.
- Wheels and tyres — non-standard alloys or tyre sizes.
- Audio and tech — upgraded ICE or speakers, aftermarket screens.
- Security and practical — alarms, immobilisers, trackers, tow bars, roof racks, parking sensors.
One useful exception: optional extras you chose from the maker's list when ordering the car new count as that car's standard specification, so they're not a separate modification. Aftermarket changes are. If you're ever unsure, the safe rule is simple — declare it.
Do you have to declare modifications?
Yes — every one, and it isn't optional. Under the Consumer Insurance (Disclosure and Representations) Act 2012 you must take reasonable care to answer your insurer's questions honestly, and modifications are squarely part of that. Declare them when you take out or renew a policy, and tell your insurer before you make a new change so you can check you're still covered.
The Financial Ombudsman Service has warned that a growing number of drivers face large bills after invalidating their cover with undeclared changes — including alterations that feel trivial, such as tinted windows or a stereo upgrade. Their advice is blunt: tell your insurer about any modification, even one made before you bought the car.
If in doubt, declare it. Read your policy wording, and if a change isn't clearly listed, ask your insurer in writing and keep their reply. Honesty costs you a slightly higher premium at worst; silence can cost you the entire claim.
What happens if you don't declare a modification?
This is where it gets expensive. If an insurer discovers an undeclared modification — usually when you come to claim — they can void the policy from the day it started, refuse the claim, and pursue you for any third-party costs, which can reach tens of thousands of pounds. The change doesn't even have to be related to the incident.
Real cases the Ombudsman has ruled on show how it plays out: a stolen car where the theft claim was refused for undeclared modifications; a policy voided after the insurer found alloy wheels three inches larger than standard; and a claim thrown out when an undeclared ECU remap and exhaust came to light — even though the owner said they were fitted before he bought the car.
It also cascades. A voided policy is logged on a national database, making future cover harder and dearer, and because you're then effectively uninsured you risk an IN10 conviction, 6 to 8 points and an unlimited fine — and your car can be seized.
Which modifications affect your premium most?
Not all changes cost the same, and a few can actually lower your premium. As a rough guide:
| Modification | Usual effect | Why |
|---|---|---|
| Engine remap / chip, turbo | Large increase | More power and speed, higher accident risk |
| Exhaust, induction / air intake | Moderate increase | Performance gain plus repair cost |
| Lowered / uprated suspension | Moderate increase | Changed handling, higher claim risk |
| Alloy wheels, body kit, spoiler | Small–moderate increase | Costlier repairs, bigger theft target |
| Custom paint, wrap, tints | Small increase | Repair cost and visibility |
| Alarm, immobiliser, tracker | Often lowers it | Reduces theft risk |
| Parking sensors | Neutral / lowers | Fewer minor knocks |
| Winter tyres | Neutral | Most insurers have agreed not to penalise them |
Note that you must declare the neutral and premium-lowering changes too — the point of declaring isn't only the price, it's keeping the policy valid.
Why do some insurers refuse modified cars?
Mainstream insurers price for the average, standard car. A modified one sits outside their model, so some load the premium heavily or simply decline. Modified parts are dearer to repair, performance changes raise accident risk, and a personalised car is a bigger theft target. None of that means you can't be covered — specialist modified-car insurers and broker panels are built for exactly this, and because they expect modifications, declaring to them often doesn't push the price up the way a mainstream insurer would.
How do you get cheaper modified car insurance?
Several levers genuinely bring the cost down:
- Declare everything accurately — the only route to a valid policy, and specialists price modified cars fairly when you do.
- Compare specialists, not just big names — the cheapest quote for a modified car is often an insurer you haven't heard of.
- Ask about agreed value — so your modifications are paid out, not just the standard-spec version of the car.
- Add security — an alarm, immobiliser or tracker can cut the premium.
- Garage it, limit mileage, raise the voluntary excess where it makes sense.
- Join an owners' or enthusiasts' club — some insurers offer members a discount.
- Get a quote before you modify — see the cost before you spend on parts and labour.
What about imported cars?
Imports raise similar questions — non-standard specification, parts availability and repair cost — so many mainstream insurers shy away from them too. Specialist import and modified insurers handle Japanese, American and European imports routinely. Declare the car's import status alongside any modifications, and compare specialists rather than assuming you can't be covered.