Revealed: the modifications that will increase your car insurance the most

Richard Aucock, Contributor
Wed, 25 Oct 2023

Even the smallest of vehicle modifications can increase car insurance premiums – and new research by Uswitch has revealed the mods that could increase premiums the most.

Analysis of 23 different modifications found car insurance premiums increased by an average of 83%, adding £624 to the annual cost of car insurance for Britain’s most popular used car, the Ford Fiesta.

However, owners cars with bored out and enlarged engines could see a staggering 384% spike in their car insurance – that’s an eye-watering £2885 premium.

Adding more power through turbocharging or supercharging is cheaper… but still sees car insurance premiums go up by over £2100 a year.

Meanwhile, fitting an engine chip that delivers more than 26bhp extra adds a hefty £1632 to a car insurance premium.

More than 1 in 10 modifications to a car in the UK involve a non-standard engine. Uswitch car insurance carried out the research, using a regular Ford Fiesta Zetec as its base model. This costs a 25-year old an average of £751 a year to insure.

Boring out the engine increased it to £3636, while turbocharging and supercharging hiked the annual cost to £2920.

More than half of UK motorists are unaware they need to contact their car insurer if they modify their vehicle, whether cosmetic or performance-based.

"It’s clear that modifying your car is likely to increase your car insurance premiums, but this may be a sacrifice you are willing to make in pursuit of your dream car," says Uswitch car insurance expert Leoni Moninska.

"It’s important to always check with your insurer what the impact of any modifications will be, and ensure any work on your vehicle is legal, and undertaken by a reputable mechanic."

Other modifications with a big potential impact on the cost of car insurance include lowering a car by more than 5cm. This could add over £1600 – which, surprisingly, is more than the £977 cost of adding a race-style roll cage.

Adding side skirts, a bonnet bulge, front spoiler or rear valence can also increase the cost of car insurance by almost £750.

There are, however, some cheaper modifications that won’t impact the cost of car insurance as much.

Fitting a non-standard exhaust only pushes up the annual cost by £5 a year. Darkened rear glass sees a modest £22 hike – and if you do want to lower your suspension, keep the drop to under 5cm and you’ll enjoy a far more modest £27 average hike in your car insurance premium.

Ask HJ

Do I need to keep my run flat tyres for insurance purposes?

I have Mercedes E350 with run flat tyres, if I replace these with ordinary tyres would this affect the insurance or my Mercedes service and breakdown cover?

It is always sensible to enquire directly with your insurer in matters such as these, as the terms and conditions may vary between providers, but we would not expect changing to standard tyres to affect your insurance. Similarly the service and breakdown cover provided by Mercedes may have terms and conditions that dictate that the car is maintained in its original specification, so we would suggest reviewing your policy documents or consulting with them directly.

Answered by David Ross

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