Car Modifications

Even if you have a current MOT, you may still be committing an offence if the condition of your car, or any of your vehicle parts, fall below the required limits. The information and guidelines below will help you to understand which modifications are legal.


To drive a vehicle at night or in poor visibility:

  • all of the necessary lights must be attached, clean and in good working order
  • your vehicle must have its headlights and rear tail lights (which must be red in colour) turned on at night. On a 30mph road which has lit street lighting, using only sidelights is legal, but it is advisable to use full headlights.
  • lights should also be used when driving in conditions where the visibility is severely reduced (where you can’t see further than 100 metres).

Please note: Night is defined as being the period between half an hour after sunset and half an hour before sunrise.

It is an offence to show a red light to the front of a vehicle (including a reflector) and a white light to the rear unless reversing. These offences are dealt with by means of a £50 non endorsable Fixed Penalty Notice. It does not matter if the lights are mounted inside or outside the vehicle, just that the light can be seen from the outside. This could also include neon lights fitted under or on the side of a vehicle and red LED windscreen washer jets. Green lights can only be fitted on Medical Practitioners vehicles.

  • Blue lights
    Only emergency vehicles can display blue flashing lights, it is an offence if your vehicle has any blue flashing LED or neon under-vehicle lighting system or lights on windscreen, washer jets or number plates emitting a blue flashing light. The motorist could receive a £50 Fixed Penalty Notice and or be reported to court.
  • Fog lights
    For vehicles fitted with front fog lights (rear fog lights are also included), it is an offence to illuminate them unless visibility is seriously reduced, which is defined as driving rain, snow or fog with visibility less than 100 metres. Fog lights cause dazzle to other drivers and can attract a £50 non endorsable Fixed Penalty Notice.
  • Hazard lights
    These lights should be used when your vehicle is stationary, to warn other road users that you are causing a temporary obstruction. Hazard lights must only be used when driving or being towed when you are on a motorway or unrestricted dual carriageway and should be used to warn drivers behind you of a hazard or obstruction ahead. They must only remain on long enough to ensure that your warning has been noticed by other drivers.
  • Daytime running lights
    These are low energy front lights that turn on automatically when the ignition is switched on, to improve visibility during daylight hours. They are compulsory on models that have gone through the European whole vehicle type approval process since February 2011 for cars and small vans and since August 2012 for lorries and buses.


Your brakes and steering will be affected by under-inflated or over-inflated tyres. Under-inflated tyres may also increase your fuel consumption. You must ensure that your tyres:

  • are inflated to the correct pressure given by your manufacturer's handbook or data
  • are free from any lumps, bulges or tears caused by a failure in the tyre's structure
  • have tread marks above the minimum depth permitted. These will vary depending on the vehicle:
  • Cars, light van, trailers - minimum depth of the tread on your tyres is 1.6 millimetres, across the central ¾ of the tread around the complete circumference of the tyre.
  • Motorcycles, passenger vehicles, large vehicles -  depth of at least 1mm throughout a continuous band measuring at least ¾ of the breadth of the tread and around the entire circumference.
  • Mopeds - tread must be visible.

Stretch tyres are illegal and could invalidate your insurance. This could lead to a prosecution of dangerous condition of vehicle, namely Sec 40A of the Road Traffic Act 1988 and is subject to a £100 Fixed Penalty Notice and three penalty points endorsed onto your driving licence or being reported to court.

The vast majority of large or big bore exhausts are illegal for use on public roads. The fact they may have passed an MOT test is irrelevant as this only checks for exhaust gasses and emission legislation compliance. However, we do have equipment that allows us to measure decibels.

Big bore and sports exhaust systems are usually fitted to increase the sound emitted and this contravenes the Type Approval of the vehicle, which is an offence.

There is no requirement for police to measure the sound level from the exhaust system, it only requires an opinion that the system is not standard and that it is noisier than a normal vehicle of the same specification.

It is not an offence to sell these exhausts systems, but it is an offence to fit one to your vehicle and drive it on a public road. Motorists who do so would be reported to court and may face a fine and court costs.

Tinted windows
Heavily-tinted windows will seriously reduce your view in certain weather conditions and most definitely at night time.

The legal requirements allowed are 75% of light through the front windscreen and 70% of light through the side window on the front doors. Any rear door glass and rear windows are not included in this requirement so they can be as heavily tinted as you like.

We do have instruments which can now measure the severity of a tinted window. If a window is found to be too heavily tinted the motorist could be subject to a prohibition notice, Fixed Penalty Notice from £50 to £100 with 3 penalty points, or reported to court.

Number plates 
If the number plates on your vehicle do not comply with the below legislation this could result in a £100 Fixed Penalty Notice being issued and/or the DVLA being notified of the offence:

  • Number plates must conform to current requirements in relation to size in terms of the number plate itself and the characters, spacing, format and font. 
  • You must not misrepresent the characters either, for example by spacing them to make up a word or name or by strategically placing fixing screw covers to alter any character.
  • Fonts including italic, bold and shadow are all illegal.
  • Number plates must be white reflective to the front and yellow reflective to the rear and the characters have to be black. The exception to this rule is if the vehicle (note: the legislation states vehicle not the number plate itself) is registered prior to 31 December 1972, in which case black and silver number plates can be fitted front and rear.

Ultimately, the DVLA can withdraw the number plate from the vehicle temporarily or permanently, even though you pay for them. The DVLA still own the plate, you are just given the privilege of displaying them on a vehicle.

Insurance Companies and Modifications

You must inform your insurance company of any modifications or changes to your vehicle, whether you carry them out yourself or if the modifications were already installed when you purchased the vehicle.

Some examples of this are any non-standard or upgraded wheels and fitment of spoilers, wings and body kits.

Failure to do so could seriously reduce any future claim you make and in a worst case scenario completely invalidate your insurance.