Front windscreen tint must let through at least 75% of light, and front side windows at least 70%, or the car fails its MOT. Exhausts must not be noticeably louder than standard, lighting must stay within approved colours (no unauthorised blue/red to the front or rear), and major changes — engine swaps, seating capacity, colour — must be reported to the DVLA to update the V5C.
MOT-relevant modification limits
| Modification | Legal limit / requirement |
|---|---|
| Front windscreen tint | Minimum 75% light transmission |
| Front side window tint | Minimum 70% light transmission |
| Exhaust noise | Must not be noticeably louder than when built; MOT testers assess subjectively |
| Lighting colour | No unauthorised colours; correct colour to front (white) and rear (red) |
| Number plates | Must follow correct font, spacing and size — no italic/stylised fonts |
Window tints
This is one of the most commonly failed MOT modification checks. The front windscreen must allow at least 75% of light through, and front side windows (the ones you and a passenger sit next to) at least 70%. There's no legal minimum for rear windows or the rear windscreen, though if there's no rear-view mirror substitute, exterior mirrors on both sides become mandatory.
Exhaust systems
Aftermarket exhausts are legal, but the car must not be noticeably noisier than it was when it left the factory, and MOT testers have discretion to fail a car for excessive or annoying noise. Since 2021, testers also check for obvious modifications like missing baffles or aftermarket systems that appear designed purely to maximise noise.
Lighting
Only white or yellow light may show to the front, and red to the rear (amber indicators are the exception). Blue and red lights are restricted to emergency service vehicles, and fitting unauthorised colours — including some underglow kits — can result in an MOT failure and, separately, a police stop.
Suspension and ride height
Lowering or raising a car's suspension is legal, but the modification must not compromise headlight aim, ground clearance for components like the exhaust, or cause geometry that leads to uneven, dangerous tyre wear — all of which an MOT tester can flag.
Number plates
Plates must use the prescribed British Standard font, correct character spacing, and standard size — stylised, italic or 3D-effect plates that don't meet the spec can fail an MOT and may also result in a separate fine, since it's a road traffic offence distinct from the MOT itself.
Tyres
Any tyre fitted must match or exceed the vehicle manufacturer's specified load index and speed rating, and all four (or at least each axle pair) generally need to be a compatible type. Mixing radial and cross-ply tyres on the same axle is illegal.
Engine changes, changes to the number of seats, and colour changes must be reported to the DVLA to update your V5C — separate from any MOT or insurance requirement. Driving without an accurate V5C on file can itself cause problems if you're ever stopped or need to sell the car.
A quick pre-MOT self-check
- Test tint levels if you're unsure — cheap tint meters are widely available.
- Listen for whether your exhaust is noticeably louder than stock.
- Check all lights show the correct colour front and rear.
- Confirm your number plate meets the standard font and spacing.
- Check tyre sidewalls for load/speed rating consistency with the manufacturer's spec.
Frequently asked questions
What's the legal window tint limit in the UK?
Can my car fail its MOT for having a loud exhaust?
Are coloured underglow lights legal in the UK?
Do I need to tell the DVLA if I change my car's engine?
Can I fit any size of aftermarket wheel and tyre?
Sources & further reading
- GOV.UK — MOT inspection manual
- Driver and Vehicle Standards Agency (DVSA)
- GOV.UK — vehicle registration and the V5C
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