What Car Modifications are Illegal in the UK?

By Grigore Spac
Updated December 30, 2020
What Car Modifications are Illegal in the UK?

Buying the car that you wish for is not always the easiest option, so people accept that they will have to make modifications to a car to improve it. But before making this there are some critical points that need to take into account before making any changes.

Increasing power, tinting windows, spoilers, engine, exhaust, bodywork. These are some of the most popular modifications people can make to their cars. But you may have some serious problems if you do some changes to your vehicle without being properly informed.

We will help you understand what car modification are illegal as there are some changes which you are not allowed to make to a car in the UK.


What is considered as a modification?

A modification is any change made to the car that amend it from the manufacturer’s standard settings. There are 2 categories of modifications: performance or cosmetic.

Performance modifications cover: altering the suspension, changing the wheels, increasing power, turbo upgrades, re-engineering the engine, chip tuning and remaps, intake and exhaust upgrades, handling modifications, lowering the suspension, fuel upgrades and transmission modifications.

Cosmetic car modifications cover: different types of stripes, body kit changes, fixing a sunroof, tinted windows, paintwork and changes to audio system.

A very important thing to remember is that if you are caught with certain modifications that are illegal, you could be charged with an offence and receive a fine. These illegal modifications are:


  • No red lights are allowed to be fitted to the front of your car.
  • No green lights at any side of the car.
  • Any kind of flashing or spinning lights.
  • No LED lights fitted to the inside of the car which did not pass European/UNECE technical standards.
  • No undercar neon lights that can not be switched off with an independent switch.
  • No tinting of rear lights that will reduce efficiency in darkness.

Window tinting

  • The front windscreen must let at least 75% of light through and the front side windows must let at least 70% of light through for cars first used after 1 April 1985 and 70% for the front windscreen and front side windows for cars first used before 1April 1985.

Important: it is illegal to sell a vehicle already fitted with glass that breaks the rules on tinted windows.

Exhaust modifications

  • It is illegal to drive a car on public roads with big bore or sport exhausts because of their excessive noise levels which is considered a public nuisance.

Catalytic converter modification

  • It is illegal to remove the catalytic converter, part of exhaust system, designed to reduce car's emissions from the internal combustion engine.
  • It is illegal to replace the catalytic converter with another one that does not comply with the applicable laws, including approval requirements governing the fitting to certain vehicles, which are set in Motor Vehicles (Replacement of Catalytic Converters and Pollution Control Devices) Regulations 2009.

Nitrous Oxide (N2O)

  • It is impossible to get a car insurance for cars fitted with Nitrous Oxide because it can increase the likelihood for a car to explode.


What modifications change the price of your insurance?

There are many changes you could make to your car and they all, no matter how minor, will have some kind of impact on the price of your insurance but we highlight those with the biggest impact.

  • Turbo or supercharger installed
  • Transmission or gear alterations
  • Exhaust system changes
  • Air filter upgrades
  • Wheel arch amendments
  • Body kit amendments
  • Spoiler additions
  • Light changes
  • Tinted windows
  • Bucket seat/racing seat installation
  • Roll bar or roll cage upgrade
  • Dashboard alterations
  • Brake replacement
  • Suspension changes
  • Alloy wheel fitting
  • Sunroof installation
  • Aircon alteration
  • Specialised paint work

To give you a better understanding of how much more the insurance will cost you for the modifications made we look at the data from Compare the Market. The price for the average top-five premium for cars with no modifications was £701.99, and for those with modifications was £782.74 (based on Compare the Market data taken from 1 June 2020 to 1 September 2020).


Why is it more expensive to insure a car with modifications?

The price of your insurance premium is based on the price of your car (with all the modifications) and on the probability of you making a claim. So, when it comes to modifications, insurance companies will consider the following details:

  • If the modifications made increase the price of your car, the insurance claims you make could be higher because it will be more expensive to repair or replace parts, especially if those parts are custom made.
  • If the modifications increased  the value of your car, it may be at greater risk of theft.
  • If you increased the power of your car and made it faster, it could increase the risk of a speed-related accident.
  • Young drivers, who usually make modifications more often, are statistically more likely to claim on their insurance, so any modifications they do could increase their premium even more.

Important: if you modify your car during the insurance you already have, you need to notify your insurance provider about all the changes which may change the price of the premium.