How do car modifications affect car insurance prices?

October 11, 2017

There are many different ways to modify a vehicle.

Some modifications can be done to improve the performance, whereas others simply make a car look better.

However, although modifications to vehicles may seem like a great idea, you must consider the affect these changes will have on your car insurance premium.

Here we explain how car modifications could affect you premium costs.

What is a car modification?

A car modification is a change or addition to a vehicle which will enhance it in some way and is not part of the manufacturer’s original factory specification.

As mentioned above, changes could improve performance, aesthetics or simply be functional modifications.

Some common examples of car modifications are:

  • Sound systems
  • Suspension
  • Personal number plates
  • Alloy wheels
  • Spoilers
  • Tinted windows


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Why is insurance affected by modifications?

Generally, most of the performance and aesthetic modifications will increase the cost of your car insurance premium. Other changes such as those made to improve safety, eg. parking sensors, won’t affect premiums and could help you avoid accidents which, over time can reduce your premiums.

When you apply for car insurance, companies will assess a number of things about your vehicle.

The three main areas of assessment are: risk of accident, risk of theft and value of your vehicle.

Below you can see why certain modifications will affect the amount you pay.

  • Risk of accident – Some car modifications improve performance and in turn, increase the risk of having an accident. Because of this, insurers will increase premiums.
  • Risk of theft – Some modifications are high in value, for example, sound systems and satellite navigation systems. Having these valuable modifications in your car will increase the risk of theft, again, increasing your premium.
  • Value of vehicle – Most modifications you make to a vehicle are not cheap to make, and due to this, the overall value of the car will increase. Insurers tend to charge more to insure vehicles which will cost more to replace or repair if they were to be in an accident.

How is car insurance affected by modifications?

Below is a list of common car modifications and the average increase/decrease in premium price due to each modification.

Engine and mechanics

  • Turbo/Supercharging                                      91% increase
  • Transmission/Gear change                             45% increase
  • Exhaust changes                                             3% increase


  • Wheel arches                                                  41% increase
  • Complete body kit                                          26% increase
  • Spoilers                                                           7% increase
  • Light changes                                                  6% increase
  • Tinted windows                                               1% increase


  • Replacement of seats                                     16% increase
  • Replacement of steering wheel                     16% increase
  • Dashboard changes                                        3% increase

Brakes and suspension

  • Uprated brakes                                               15% increase
  • Suspension                                                      2% increase


  • Specialised paintwork                                     15% increase
  • Stripes and badges                                          9% increase


  • Alloy wheels                                                    1% increase

Fuel economy

  • LPG conversion                                               no change

Driver aids

  • Satellite navigation system                             13% increase
  • Car phone kit                                                  12% increase
  • Parking sensors                                               13% decrease



Notifying your insurance company

When searching for car insurance you must always inform your insurers of any modifications made to your vehicle. Even if you purchased the car with these changes. Failure to do so may result in your policy not paying out.

If you already have a policy in place and decide to make changes to your car, you must let your insurance company know as soon as you do so. Do not wait until it is time to renew, as again, your policy could be invalidated.

Illegal Modification

It is worth noting that not all car modifications are legal. Making illegal changes to your car may result in you being prosecuted, as well as your car insurance policy being invalidated.

You should always seek expert advice before modifying your car to avoid being caught out.


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Some examples of illegal car modifications are:

Tinted windows – Car windscreen must let in 75% light and front side windows must let in 70%.

Number plates – You are permitted to have a personalised number plate, however you must adhere to the law. For example, you must use the correct font and spacing.

Why car modifications may not be the best idea…

As well as the risk of increased car insurance premiums, modifying your car can cause a lot of other detrimental effects. We’ve listed a few of them below just so you’re aware of some of the potential issues you could run into:

Fuel economy

As modified vehicles tend to be heavier – especially those with bigger wheels fitted – this will negatively affect the fuel economy of your car, meaning you’ll be visiting the petrol/diesel pumps a lot more!

Sports exhaust have the same effect. You may get heads turning with that loud exhaust while you cruise about town, but the overall performance of the car will be affected.


Your bigger wheels can also have an impact on how well your car manages on the road – which is a cause for concern when you factor in the notoriously poor state of many roads in the UK.

Many modified wheels are also fitted with low profile tyres, which means they are a lot more likely to suffer punctures.

Noise and emission guidelines

You know that part of your MOT test that deals with the noise and emissions output of your car? Well, modified cars could lead to you failing that part of the test due to producing too much noise and exceeding the emissions levels.

How to tell if a car has been modified

With many modified cars, it is obvious that they have been ‘upgraded’ by the owner (the huge exhausts, shiny rims and massive spoilers can be a giveaway). However, it’s not always that easy to spot – so if you are thinking about buying a second-hand car here’s a few tips to help you spot any modifications:

Ask the owner – an obvious tip, but you may not always get an honest answer

Common car mods to look out for/ask about are:

  • Bigger wheels
  • Alloy wheels
  • Adjusted suspension
  • Sports exhausts
  • Spoilers
  • Tinted windows
  • Upgraded stereo systems

If you suspect that any of the above are not ‘factory-fitted’, and you are not convinced by the current car owners insistence that they are, then refer to the vehicles specifications (which can easily be found online). Parkers is always a good place to start:

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Here at Bobatoo, Bob knows that insuring your car can be confusing, so he wants to make sure that you have all the information you need to get the best out of your policy. Read our latest car insurance guides here:

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