There are a myriad ways to alter the appearance and performance of your car, but some of these modifications could affect your vehicle's safety and handling. In order that particular safety standards are met, the UK has in place various laws to ensure modifications do not put the lives of drivers or passengers at risk.

Here we look at some of the key aspects of the law in relation to car modifications.


A number of high performance exhaust system upgrades are available which are effective at increasing your engine's performance. It is important to recognise, however, that most big-bore exhausts are not legal on UK roads due to excessive noise and emissions. Keep exhaust silencers in place if you do upgrade your exhaust.

Costs: Most exhaust systems range between £500 and £1000, but can cost more.



Among the most common brake modifications are larger brake callipers, along with the fitting of a variety of vented, drilled or grooved brake discs. In order to ensure the suitability of any new brakes, it's advisable to seek assistance from a professional engineer and obtain advice from the car manufacturer or certified suppliers. Upgrading or modifying your braking system should not mean you begin braking later or start taking corners at higher speeds, since both may imperil you and your passengers.

Costs: The price of big brake kits vary greatly, depending on your car. They can range from several hundred pounds to several thousand.


Improved handling and a sportier appearance are the two main attractions of adding spoilers to a car. Any spoiler must be securely fitted and not have any dangerously sharp edges; it must also not obstruct your view.

Costs: How much you pay for a spoiler depends on its size and which model it is intended for; a small one may cost as little as £100, with prices for more elaborate ones easily surpassing £1000.

Number plates

Personalised number plates are increasingly popular in Britain, but if you change your registration you must bear in mind that:

  • Number plates must be clear of any obstruction and must not mislead other drivers and road users.
  • Legislation regarding letter/digit size, font, characters, spacing and format must be adhered to.
  • Front plates must be reflective white; rear plates reflective yellow.
  • £100 Fixed Penalty Notice may be issued to those who fail to comply.

Costs: The DVLA offer personalised number plates starting at £399. Search for available plates here:

Undercar Lights

Thinking of placing neon lights under your car? If so, you need to ensure the tubing is not on show in order to comply with the law. Lights must not be too bright, since they might distract other road users. Adding an on/off switch is prudent so they can be turned off easily in inappropriate situations.

Costs: there are a vast range of LED lights available, with prices starting at a few pounds for small light set ups.

Headlights/fog lights

Any upgraded/modified head lamps or fog lights must be white or yellow at the front and red at the rear – no other colour is allowed.

What about blue lights?

Only emergency services are permitted to use blue lights.

More information on legislation regarding vehicle lights:

Tinted windows

The law permits tinted windows only within certain parameters: rear side windows and rear windscreen can be tinted without restriction, but the windscreen and front side windows must allow light transmission of 75% and 70% respectively. Failure to comply with these rules could mean you are served with a Prohibition Notice, meaning you cannot use your car until the windows meet regulations. A penalty notice or even a court summons are also possibilities if you are caught.

Costs: prices can range from £100 to well over £1000, depending on how many windows are being tinted and which tinting system is being used.


It is best only to fit wheels that your car’s manufacturer has recommended or approved, since changing your wheels can greatly affect handling.

Costs:  Prices vary but sets are available for under £200.


Ensure there is ample space between any new tyres and the arches, taking into account suspension movement. This will also ensure the tyres are unaffected when the steering is in full lock.

Costs: Again, costs vary hugely, with budget options starting under £40 per tyre, rising to hundreds for high performance options.

Telling your insurer about any modifications

Whether modified by you or a previous owner, you must tell your insurer about any and all modifications. If you do not, any future claim may be repudiated – and your insurance policy could be invalidated.

How will modifications affect resale value?

While fun and rewarding to you, your car modifications may not be to the taste of others. Someone somewhere may appreciate that ear-piercing exhaust or those funky aftermarket wheels, but it may take longer for them to find your car when the time comes to sell.

In the meantime, you may find yourself dropping the asking price if you need to make a sale quickly. When it comes to cars, ‘normal’ and ‘innocuous’ are the watchwords for a speedy deal.