Modifying cars: what can you and can’t you do?

Modifying cars

Once upon a time this would have been a humble family runabout…

We all like to give things our personal touch and modifying cars is no different. But while it might please you to make parts of your car bigger, brighter, faster and louder, it could land you in hot water.

For a start, the law takes a dim view of cars that aren’t considered roadworthy. And insurers may even refuse to pay out if you modify a car without telling them. Here we look at what you can and can’t do to your car. And whatever you decide, make sure you do it with safety in mind and that you inform your insurer.

In-car entertainment

The ability for drivers to amuse themselves on the move has boomed over the past few years. And there’s nothing to stop car owners having as many screens in their machine as they want. What they must remember is that there shouldn’t be anything that can distract them from the primary task of driving.

it’s illegal to have moving images within view of the driver. That means you can’t have a screen showing video that you can see through the rear-view mirror or that’s mounted on the dash. And the laws about not touching mobile phones apply to any other mobile devices.

Wheels and tyres

The first thing car designers do when making sketches of new models is to sit them on over-sized wheels. This is because cars with big wheels look sportier and more purposeful. In theory, putting bigger wheels on is a simple modification. The practice is very different.

Bigger wheels can adversely affect how a car handles. Instruments will likely require recalibrating. And larger wheels and tyres might foul suspension components or wheel arches. If you do want bigger wheels, make sure they’re approved by your vehicle’s manufacturer and always tell your insurer.

Equally, messing with tyres can cause trouble. There’s a trend called ‘eurofit’ where smaller tyres are stretched over large wheels. Tyre manufacturers don’t recommend it as the technique can prompt premature tyre failures.


Cars that sit on lower suspension invariably look sporty. However, lowering a car’s suspension can have a detrimental effect on the way it handles. That’s in addition to the negative impact it will have on the ride. If you must have it done, make sure you get a qualified automotive engineer to do the work.


Modifying cars

We’ve all seen cars with neon lights beneath them, blue lights in their headlamps and even on washer jets. But are they legal? The answer is yes. And no. The Road Vehicle Lighting Regulations 1989 state that it’s illegal for a regular passenger car to show any light other than red to the rear (apart from reversing lights, turn indicators and reg number lights).

The law isn’t so clear at the front. Front position headlamps must be predominantly white or yellow. But there’s nothing strictly outlawing blue lights in other locations. That said, the law is clear that only emergency vehicles are allowed blue flashing lights.


Uprating brakes is a popular modification to cars. Sporty brake disc and caliper kits that are supposed to improve braking performance are available from retailers. Assuming they don’t impair your car’s stopping capability, there’s nothing wrong with these. But make sure that they’re suitable for your car and won’t require any modification to your car’s mounting points to fit them.


What a car sounds like can make the difference between racy and run of the mill. However, there are limitations to what you can do. And it all revolves around noise. Having a drainpipe-sized exhaust might make the uninitiated think your four-cylinder hatchback has a throbbing V8 beneath the bonnet. That’s because the noise it makes will be breaking the law and it may well cause a deterioration in the exhaust emissions your car pumps out. If you do have a new exhaust fitted, make sure the silencers are in place.

Number plates

A simple way to stand out from the crowd is to have a personalised number plate. What about having a plate that rearranges the spacing of letters and numbers to say a word, your name perhaps? Or letters in one of those fancy curly fonts? The government website is adamant: “You can’t rearrange letters or numbers, or alter them so that they’re hard to read.” And then it says in big letters that incorrectly displayed number plates could incur a £1000 fine.


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