Is a Cat-Back Exhaust Legal in the UK on a BMW M4?
A cat-back exhaust is generally road-legal on a BMW M4, as long as it keeps the catalytic converter and is not noticeably louder than standard.
The rule itself is the same whatever you drive: the law in the UK does not single out the BMW M4. What changes from car to car is which cat-back exhaust systems actually fit and how they are certified, so that is what this page covers. For the rule in full, see Cat-Back Exhaust law in the UK.
Why?
A cat-back system on the BMW M4 replaces the pipework and silencer behind the catalytic converter, so the emissions equipment the car was type-approved with stays in place. Because the cat is untouched, the car can still meet the exhaust-emissions standard it was built to, which is the part the law cares about most.
The real limit is noise, not the parts. Regulation 54 of the Construction and Use Regulations says a vehicle must keep its exhaust silencer and must not have the exhaust altered to increase the noise it makes beyond its type-approved level. At MOT, the "nuisance" check fails an exhaust that is clearly louder than a standard system in average condition.
So a quality cat-back that retains the catalytic converter and isn't excessively loud is generally legal on the BMW M4. A straight-through or de-cat setup, or anything that makes the car noticeably louder, is not.
What decides if it's legal
- Catalytic converter stays fitted (a cat-back connects after the cat).
- Not altered to increase noise beyond type-approval (C&U reg. 54).
- Still passes the MOT noise/nuisance and emissions checks.
- Declare the change to your insurer.
Popular cat-back exhaust for the BMW M4
Related guides
Related legality guides
Sources
- Road Vehicles (Construction and Use) Regulations 1986, reg. 54
- MOT inspection manual: 8. Nuisance (GOV.UK)
- Catalytic converters and diesel particulate filters (GOV.UK)
Sources last reviewed July 2026.
This page is general guidance, not legal advice, and reflects the rules in the UK for a typical BMW M4. Confirm with the manufacturer, your insurer and the latest DVSA and GOV.UK guidance before modifying.