Changing the look or sound of a car is a popular hobby, and many modifications are perfectly legal when they are done properly. This guide is for new drivers who are thinking about modifying a first car, or who have bought one that has already been modified. It covers what to tell your insurer, the legal limits on window tints, exhausts and lights, and what each change can mean for the MOT.
Telling your insurer: the legal duty
UK-wide: the Consumer Insurance (Disclosure and Representations) Act 2012 says it is the duty of the consumer to take reasonable care not to make a misrepresentation to the insurer. This applies before a policy is taken out and when it is changed.
The consequences depend on how the mistake happened, according to the Act:
- Deliberate or reckless: the insurer may cancel the policy from the start, refuse all claims, and need not return the premiums unless keeping them would be unfair.
- Careless: the outcome depends on what the insurer would have done if it had known. If it would not have insured you at all, it can refuse all claims but must return your premiums. If it would have charged more, it can reduce a claim payment in proportion.
The Association of British Insurers (ABI) notes that modifications and aftermarket rims can make a car more expensive to insure, and warns that not having the right cover may invalidate a policy. nidirect puts it plainly: some modifications could put you on the wrong side of the law “and perhaps your insurance company”. Check your policy documents for when changes must be reported, and ask before you modify, not after.
What might count as a modification
UK-wide: insurers set their own definitions, so the safe approach is to ask about any change from the car’s original specification. nidirect’s guide to modified cars covers in-car entertainment, lights and reflectors, wheels and tyres, suspension, brakes, tinted windows, spoilers, number plates, exhausts and emissions equipment. Some changes also need to be recorded with the DVLA.
| Change | What official guidance says |
|---|---|
| Wheels and tyres | nidirect says changes can significantly alter how the car behaves, and replacements should be approved by the manufacturer with adequate clearance from the bodywork. |
| Suspension | nidirect says stiffer springs and lowering kits should only be fitted where road handling will not be compromised, by a proficient person. |
| Engine remapping | nidirect says changing control mapping to increase smoke, noise or emissions, or to bypass emissions controls, counts as tampering. |
| Colour, engine, cylinder capacity, fuel type, seating | GOV.UK says these changes must be recorded on the V5C log book (GB). |
nidirect also reminds owners that you are responsible for making sure the vehicle is legal and safe at all times, regardless of who carried out the modifications. That includes a car you bought already modified.
Tinted windows
Great Britain: GOV.UK says that for vehicles first used on or after 1 April 1985, the front windscreen must let at least 75% of light through and the front side windows at least 70%. For vehicles first used before that date, both must let at least 70% through. There are no rules for the rear windscreen or rear passenger windows. It is illegal to fit or sell glass, or a vehicle, that breaks the rules. The police and DVSA examiners use light measuring equipment, and you could get a prohibition notice stopping you using the vehicle until the extra tint is removed, a penalty notice or a court summons.
Northern Ireland: nidirect gives the same 75% and 70% figures for most cars. The police and Driver & Vehicle Agency (DVA) examiners measure tints, and the possible outcomes are a prohibition notice, a fixed penalty notice or a court summons. nidirect adds that most modern cars already have a slight factory tint, so adding more is likely to take the windows below the legal limit. Sun strips must not encroach on the area swept by the original wipers.
MOT (GB): the MOT inspection manual tells testers to fail excessive tinting only where the driver’s view is significantly affected. An MOT pass is therefore not proof that a tint meets the legal light limits.
Exhausts and noise
Great Britain: GOV.UK says it is illegal to modify the exhaust system to make a vehicle noisier after it has been type approved. The police can also act if the silencer does not work as designed, or if you drive in a way that creates too much noise.
Northern Ireland: nidirect says all exhaust silencers must be kept in good working order, and you will be breaking the law if you remove a silencer or make any change that makes the vehicle louder than the original exhaust. Emission control equipment must not be removed, bypassed or tampered with.
MOT (GB): testers judge whether exhaust noise is unreasonably above what a similar vehicle with a standard silencer in average condition would make. Exhaust noise above permitted levels is a major defect, which means an MOT fail. A vehicle emitting dense blue or clearly visible black smoke also fails.
Lights
Great Britain: the Highway Code says you must not use lights in a way that would dazzle or cause discomfort to other road users, and that lights must be properly adjusted to prevent dazzling. Under the MOT inspection manual:
- headlamps must emit white, predominantly white with a blue tinge, or yellow light
- halogen headlamp units on vehicles first used on or after 1 April 1986 must not be converted to use HID or LED bulbs; a converted headlamp fails
- a complete replacement headlamp unit built with HID or LED light sources is acceptable if it meets the other requirements
- rear position lamps must emit a steady red light, and a lamp showing red to the front or white to the rear is a defect
Northern Ireland: nidirect says headlamps must show a largely white or yellow light, rear position lamps a steady red light to the rear only, and tinting rear reflectors in a way that reduces their efficiency is not permitted. It also warns that many aftermarket LED bulbs are not road legal and can cause dazzle and glare.
The safety and your vehicle topic and the show me, tell me questions both cover lights and vehicle checks.
What it means for the MOT
Great Britain: GOV.UK says a vehicle fails its MOT if the result lists any “dangerous” or “major” problems, and you might not be allowed to drive it until they are fixed. Driving a vehicle that failed because of a “dangerous” problem can bring a fine of up to £2,500, a driving ban and 3 penalty points.
Northern Ireland: GOV.UK notes that vehicles registered in Northern Ireland follow different MOT rules and processes, run by the DVA.
Quick-fire questions
Is the tint my car came with from the factory legal? The Highway Code says tinting applied during manufacture complies with the light transmission standards. nidirect warns that adding more tint on top is likely to take the windows below the legal requirements.
Can I fit LED bulbs in my halogen headlights? For vehicles first used on or after 1 April 1986, the MOT inspection manual says a halogen headlamp converted to HID or LED bulbs fails the MOT. nidirect warns that many aftermarket LED bulbs are not road legal.
Before you go
Modifying a car can be done legally, as long as the changes stay within the rules, and your insurer needs to know about them. Ask your insurer before making any change, keep front tints, exhaust noise and lights within the limits, and update the V5C where GOV.UK requires it.
This is general information, not legal advice.
Sources
- legislation.gov.uk, “Consumer Insurance (Disclosure and Representations) Act 2012” (legislation.gov.uk)
- Association of British Insurers, “Reducing the cost of motor insurance” (abi.org.uk)
- Association of British Insurers, “Motor insurance” (abi.org.uk)
- nidirect, “Modified cars – what you need to know” (nidirect.gov.uk)
- nidirect, “Tinted vehicle windows – the rules” (nidirect.gov.uk)
- GOV.UK, “Tinted vehicle windows: the law” (gov.uk)
- GOV.UK, “Noise from roads, trains or planes” (gov.uk)
- GOV.UK, “Change vehicle details on a V5C registration certificate (log book)” (gov.uk)
- GOV.UK, “Getting an MOT” (gov.uk)
- GOV.UK, “MOT inspection manual: cars and passenger vehicles” (gov.uk)
- GOV.UK, “The Highway Code: Annex 6. Vehicle maintenance, safety and security” (gov.uk)
- GOV.UK, “The Highway Code: General rules, techniques and advice for all drivers and riders (103 to 158)” (gov.uk)