Modifying your car: UK laws, insurance and tips (2025)

Car modification, also known as car tuning or car styling, is a change made to a vehicle so that it no longer matches the manufacturer’s original specifications.
Car modification could involve: • Performance modifications to tune the car’s performance. like engine tuning, performance-enhancing brakes, steering, and suspension • Aesthetics changes like new alloy wheels, spoilers, colour changes, new headlights or grille • Safety modifications like parking sensors • Other modifications like tinted windows, neon lights or enhanced sound systems Car modification isn't the same as repairing a car, because a car repair replaces the damaged car part with an identical one or fixes it to maintain the car according to factory standards. Car modification is done to customise your car, to make it look more stylish or to enhance handling and performance using bespoke car parts.

There are two main types of car modifications:
1. Cosmetic modification 2. Performance modification The most common modifications include increasing horsepower, tinting windows or lights, adding alloy wheels, spoilers, exhausts or engine chrome covers, or making customisations to interiors and bodywork. Let's look at them in more detail.

Changes made to the car purely for its looks and aesthetics are classed as cosmetic modifications. Cosmetic modifications can include changes to:
• Body kit • Front and rear bumpers • Rooftop vents • Paintwork • Wheel accessories • Interior LED lighting • Light lens tints • Tinted windscreens • Back bumper reflector lights • Custom hood ornament • Dashboard customisation These are some great ways of making your car stand out in the crowd. You could choose from a simple paint job to give your car a distinctive colour, or go all in and get a number of different customisations to make your car uniquely yours. What are xenon and LED car lights?

Performance modifications help improve the handling, fuel efficiency, power, and speed of the car. Some common areas of upgrades are:
1. Brakes 2. Engine and drivetrain 3. Exhaust 4. Suspension 5. Steering 6. Filtration These modifications change your car subtly – you’ll feel your car’s improved overall speed and performance while the car will continue to look the same as others.

Some car modifications are illegal in the UK because they can be unsafe for the driver and others around them. You can be at risk of a fine if you make these modifications to your car, which is why it’s important to keep the law in mind before getting any changes to your vehicle.
Some common car modifications that are illegal in the UK are:

1. Neon Lights

Neon light modifications are illegal in most scenarios. One of the few ways you can legally spruce up your car with neon lights is if you add them to the bottom of your car – but only if the neon tubing isn’t visible, and the light isn’t too bright or flashy that it distracts other drivers around you.

2. Rear and headlight tints

In the UK, white and yellow lights are allowed at the front, while red lights are allowed for the back of the car. Using red, green or purple tints for your headlights, or any bright neon or flashy lights on the exterior or interior of the car, are sure to get you pulled over. Any halogen bulbs with a colour temperature over 4,200 K are not road legal as they produce a slight blue tint light. Blue flashing lights are used by emergency services such as police vehicles, ambulances, firefighters, and the coastguard. To make these services stand-out, these lights are legal only for emergency service vehicles. Also, the lights shouldn’t be dimmed by more than 50% to maintain visibility. If the light bulbs bear an ECE R37 regulation, it’s a sign that the lights are road legal.

3. Window tints

You can tint your windows, but only to a certain degree. The front windscreen needs to be at least 75% tint-free and the side windows need to be minimum 70% tint-free to ensure a clear view of the surroundings. The only exception is if your vehicle was first put on the road before 1 April 1985. In this case, both the front and side windows need to be 75% tint-free. It’s illegal to sell a car that doesn’t follow these rules.

4. Loud exhausts

A loud, roaring car sure sounds thrilling, but the noise might irritate others. An exhaust cannot cross the noise limit of 74 decibels, so any modifications that breach this limit are illegal. All cars are ‘type approved’ under the Individual Vehicle Approval (IVA) scheme before they’re sold to you, which means that they comply with the emissions and safety regulations. Other exhaust modifications, like changes to the exhaust management system, tampering with the emissions control system, or removing some parts could make the vehicles more damaging to the environment.

5. Spoiler upgrades

Spoilers help with aerodynamic efficiency, and a slight tweak to their look can help improve handling and make the car look sportier. While modifying a spoiler isn’t illegal, the method of modification can be. If the new spoiler isn’t fastened safely to the car or has exposed sharp edges, you can get into trouble with the police and may even be forced to remove the modified spoiler. Spoiler modifications should help reduce your car’s drag. A rear spoiler can improve the downforce, while front spoilers can reduce drag. The modification should also be a practical size so it doesn’t obstruct your view – three inches wide and six inches above the roofline spoilers are usually considered an efficient size.

6. Nitrous Oxide engine modification

The use of Nitrous Oxide is illegal and extremely dangerous. Adding Nitrous Oxide gas to your engine can increase cylinder pressure. If the engine’s components are unable to handle the increased stress, the engine can suffer damage. The engine can also be damaged when there’s too much oxygen and not enough fuel in the engine (an incorrect ratio of air and fuel). In the worst-case scenario, the incorrect ratio and added cylinder pressure could cause the engine to explode.

Even the slightest change to your car will have an impact on the insurance cost. Your insurance provider may think that increasing your car’s performance can mean a higher risk of accidents. Changing the appearance of your car to make head turns can also attract thieves, which could mean there’s a higher risk of your car getting stolen. In any case, a higher risk usually translates into a higher premium charged by insurance providers.
Once you’ve decided to get a car modification, you need to inform your car insurer about the changes, no matter how small they are. It might pinch to pay a higher premium because of the modifications, but it’s necessary as your insurance coverage can become invalid if you fail to inform them. If you need to make a claim or if someone files a claim against you and you haven’t informed the provider, you may not be eligible for compensation. Not all modifications lead to a rise in your premium. Some might bring down the total amount you pay, like engine downsizing or adding high-performance brakes.

Could increase your premium:
• Supercharger installation: these can increase engine power, raising the risk of accidents and higher insurance costs. • Brake discs upgrades such as carbon-ceramic discs or drilled discs: performance-enhancing modifications can lead to higher premiums due to potential safety concerns. • Engine control unit (ECU) reprogramming to tune engine performance: these boost performance but can increase the likelihood of higher speeds and riskier driving. • Body kits to improve safety: modifications to bodywork can increase potential repair costs, which can raise premiums. • Exterior cosmetic modifications such as flared wings, wheel arches, spoilers, valances, paint jobs, tints, stripes, and badges: all of these aesthetic changes can make cars more attractive to thieves and increase repair costs. • Interior cosmetic modifications such as changes to upholstery, sat-nav systems, dashboard, phone kits, air conditioning: custom interiors add value to the car, making it more expensive to repair or replace. • Short shift kits: these can enhance transmission, potentially leading to higher performance and increasing the risk of accidents. • Cold air intake filter installation for increased power: these increase the engine output, potentially leading to riskier driving and higher premiums. • Sunroof: often adds value to the car, making it more costly to repair or replace, so driving up insurance premiums. • Alloy wheels: can increase the car's value and theft risk, leading to higher insurance costs. Could decrease your premium: • Adding technology like immobilisers, smart water, and parking sensors: security and safety features reduce the risk of theft and accidents, potentially lowering premiums. • Anti-roll bars: these improve vehicle stability, which can reduce the likelihood of accidents and lower premiums. • Engine downsizing if it increases car efficiency: a smaller, more efficient engine may lower risk and reduce insurance costs. • Improved braking system: this can enhance safety by reducing stopping distances, potentially decreasing the risk of accidents and lowering premiums. • Suspension upgrades if it improves handling: as better handling can reduce the risk of accidents, your car could be seen as safer - potentially lowering insurance costs.

Buying a customised car can have its pros and cons:

Pros:

• Internal modifications could have improved performance • You can have a unique-looking car at a low or no extra cost • If you personally like modifications, then you get them without having spent any money or time on it

Cons:

• The modifications might be damaging the car’s engine • Risk of the modifications not being done by a professional • Warranty might be void because of the modifications • The fitted parts might be low quality • Legality issues, as mentioned above If you’re considering buying a modified car, you should collect information like where the modification was done, which brand’s car parts were fitted, any mechanical issues and the value of the car after the modifications. Also ask for a service history of the car. Selling a car with modifications is allowed if the modifications are legal in the UK. It can be a good idea to retain the receipts for the modifications you buy. This shows the buyer where you got the modifications done and the quality of the parts used.

Under a PCP car finance contract, you aren’t the legal owner of the car until you’ve paid the full amount. This means you can’t modify a car bought on finance unless you get written permission from your finance provider.
If you’re able to get permission for any modifications, it’s important that you only modify what has been agreed to in the contract. If you change anything that hasn’t been agreed upon, you’ll be in breach of the contract and may become liable to pay excessive penalties and fees or have your agreement terminated. it's also important to consider what your finance provider would consider being a modification. You might think that getting an upgraded stereo or sat-nav system or any temporary changes are not actually modifying the car, but your lender may think otherwise. In all instances, check with your finance provider before you start making modifications.

If you’re keen to modify your car, then here are some key points to get started:
• Check if your intended modifications are legal • Check you're the car's legal owner • Inform your insurer and get a specialist car modification insurance • Find out how the modification will affect your car’s performance • Know if the modification can be reversed or not, and its cost • Keep in mind any risks associated with the modification • Go to a professional to get the changes done Looking for something a bit different? On Auto Trader, you can easily search for modified cars.

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