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.


14 comments on “Modifying cars: what can you and can’t you do?

  1. Brian Watts (Expert on Morgan three wheelers.) 11/04/2018 8:05 PM

    A Nick Shaw on a restoration TV programme displayed a sense of humour usually associated with a slow teenager. Spoke of the famous Morgan cars being “a wheel short” and the model F4 in production for 18 years having the (Rubery Owen) chassis made up from scrap metal.

    The one I saw was said to be a presenter so I hope your Editor is someone else.

    • Mike Shaw 18/04/2018 10:28 AM

      Hi Brian. Rest easy, Nick Shaw is someone else entirely.

  2. Robert 13/04/2018 9:58 AM

    Those who modify their vehicles and do not declare the modifications to their insurance companies are likely to find that they are uninsured. Perhaps when police check such vehicles they should ask the question of the insurers “have any modificatiuons been declared?”
    Uninsured vehicles are likely to be seized & crushed.

  3. Sandra Shaw 13/04/2018 1:16 PM

    I note your info. regarding exhaust modifications.
    Sadly the police do not appear to be interested!

    • Anthony Sansom 18/04/2018 8:03 PM

      They should be! Fitting an illegally loud exhaust tends to go with a generally irresponsible attitude to driving and danger to themselves and others.

  4. bill harding 13/04/2018 8:00 PM

    hi thanks one thing i must state is exhaust row I’m reg disabled and walk very slow crossing any road and .and they will sit there and rev .i also walk a dog and they upset him with the row.why is it that this country allow this row ,other countries don’t. i hope this makes sense, Freddie ,

    • David 17/04/2018 8:18 PM

      They’re not ‘allowed’ to rev, but unless there’s a copper there to see the incident, boy racers will get away with it. Same as street racing or illegal car mods – people sometimes do things they aren’t allowed to do.

  5. Jonathan 17/04/2018 7:34 PM

    As for moving video in front of the driver what about the large moving display on the toyota prius that most taxi drivers now use.

  6. Robert McLoughlin 18/04/2018 8:52 PM

    There should be a regulation against excessive idling of engines so that the snowflakes among us can have their cars air conditioned or still warm when they enter.

  7. Neil 19/04/2018 8:34 PM

    What about tinting your windows

  8. Michael Coyne 20/04/2018 9:28 AM

    Is using dash cams and rear view cams classed as a modification?
    I “think” I have read somewhere that such cameras are against the rules in some Euro places……is this right?

    • Sam 23/04/2018 3:52 PM

      I’m pretty sure it is not considered a modification but if you let your insurer know that you do have dashcam(s) fitted to your car then it can bring your insurance down. Just make sure to actually have one and ensure that it is working or else your insurance can be void.

  9. Sarge 22/04/2018 12:31 AM

    Green flag, did no-one teach you at school 5 hat you NEVER start a sentence with And? If you a going to lay down the law, use the English language correctly.

  10. Dean Stirling 05/09/2019 6:00 PM

    Im seeing allot of cars with tinted number plates now… Whats the legality on this?

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