Underside Corrosion and MOT: The 30cm Prescribed Area Rule
Quick Answer
Corrosion within 30 cm of any structural mounting point — suspension, seatbelts, fuel system, exhaust supports — is judged under the strictest MOT rules. Holes through metal, scaly rust that flakes when prodded, or any structural weakening at those locations is a major fail. Welding repair averages £80-300 per panel; rotten rear axles can write a car off.
Salt-laden UK roads make underbody corrosion the silent killer of older cars. The MOT Inspection Manual draws a clear line: rust within a defined prescribed area around structural mounts is treated harder than rust anywhere else. Knowing that 30 cm boundary can save you a costly fail. Run any used car through our free MOT history check before buying to spot a history of corrosion advisories.
Will it fail your MOT? The exact rule
Appendix A of the DVSA MOT Inspection Manual defines a 'prescribed area' as any structural component within 30 cm of a body mounting, suspension mounting, steering attachment, seatbelt anchorage or fuel system mount. The strictest assessment criteria apply inside this zone.
Within the prescribed area, any of three conditions is a major fail: a hole through load-bearing metal, structural weakening that gives under hand or hammer pressure, or scaly rust that flakes off when scraped. Outside the prescribed area, surface rust without weakening is generally an advisory.
- Hole through metal in prescribed area: major fail
- Weakening (gives under pressure): major fail
- Scaly/flaking rust in prescribed area: major fail
- Surface rust outside prescribed area: advisory
- Repair plates not properly welded: major fail
Where the prescribed areas are
Most failures cluster around four hotspots. The rear axle/torsion beam mounting points (notorious on Vauxhall Corsa D, Astra J and Astra H), the front subframe mounts, the rear seatbelt anchorages and the inner sill where the jacking point is welded.
Lesser-known prescribed areas include the fuel tank straps, exhaust mount brackets and brake pipe support plates. A solid-looking sill can hide rotten metal underneath the plastic cover, which is why a torch and screwdriver inspection beats a cursory walkaround.
- Rear axle/torsion beam to body mounts
- Front subframe attachment points
- Rear seatbelt anchorages and floor
- Inner sill where jacking points weld in
- Fuel tank strap mounts and filler neck
- Exhaust hangers within 30 cm of body
How testers actually check
DVSA testers use the 'corrosion assessment tool' (a small toffee-hammer-like device) to tap at suspect areas. Sound metal returns a sharp clear note. Corroded metal sounds dull, gives under pressure or flakes audibly.
Section 6.1.1 of the manual specifies the test pressure as 'reasonable hand pressure'. Testers cannot punch holes in your car, but they can probe scaly areas with the tool and note any flaking that exposes fresh rust beneath.
How to spot it before the test
You don't need a ramp to do a useful pre-MOT corrosion check. With the car parked on level ground, lie at each corner with a torch and look at the inner sill, the rear axle-to-body mounts and the seatbelt anchor points. Tap each spot with the back of a screwdriver — sound metal pings, corroded metal thuds.
Lift the boot floor and inspect the spare wheel well drain plugs. Pooled water sits there for years and rusts the floor pan from underneath. Any visible perforation here is a fail because the rear seatbelt mounts are within the 30 cm zone on most hatchbacks.
Cost to fix before MOT day
Welding repairs are quoted by panel size and access. A small patch over a sill jacking point at an independent welder is typically £80-150. A rear axle mount repair on a Corsa D runs £150-300 per side because the axle has to be lowered.
Some failures are economic write-offs. Rotten rear axle replacement on a Vauxhall is £400-700 fitted second-hand, which can exceed the value of an older example. Always get a quote before committing to repair on a sub-£2,000 car.
- Small sill patch weld: £80-150
- Rear seatbelt mount repair: £100-250
- Subframe mount repair: £200-400
- Rear axle mount repair (per side): £150-300
- Used axle replacement: £400-700 fitted
MOT history clues for buyers
Corrosion advisories are an early warning that becomes a fail within 1-2 MOT cycles. Run any used reg through our free MOT history check and search for 'corrosion', 'corroded', 'excessively corroded' or 'prescribed area'.
Cross-check our common MOT faults database for the model. Vauxhall Corsa D rear axle, Ford Fiesta Mk6 rear sill, Mini R56 boot floor and BMW 1 Series E87 rear subframe are well-documented prescribed-area failure points.
Repair best practice and pitfalls
MOT-acceptable corrosion repair must use continuous seam welding to a full-strength steel plate of at least the same thickness as the original. Pop-rivets, fibreglass and chemical metal compound are explicit fail items.
Underseal applied over rust merely hides the problem and does not satisfy a tester. If the metal beneath is unsound, the only acceptable fix is cut, weld and seal — usually a half-day job at a specialist.
Frequently Asked Questions
How big does a hole need to be to fail an MOT?
Any hole through load-bearing metal in the prescribed area is a fail, regardless of size. Outside the prescribed area, holes still fail if they affect structural integrity, but cosmetic perforation alone may pass as an advisory.
Can I patch over rust with fibreglass or filler?
No. Section 6.1.1 explicitly disallows non-structural repairs to load-bearing parts. Acceptable repairs require continuous-seam welding of a steel plate of equivalent thickness.
What is the 30 cm rule exactly?
The prescribed area extends 30 cm in every direction from any structural mounting — suspension, steering, seatbelts, body mounts and fuel system. Corrosion within that radius is judged to the strictest standard.
My car has scaly rust on the underside but no holes — will it pass?
It depends where. Scaly rust outside a prescribed area that flakes only lightly is usually an advisory. Inside the 30 cm zone, scaly rust is treated as structural compromise and is a major fail.
How much does corrosion repair cost in the UK?
Sill patches start at £80-150, rear axle mount repairs run £150-300 per side, and subframe repairs reach £400. Always weigh the cost against the car's market value before committing.
Inspect the prescribed areas yourself a fortnight before your test, get borderline rust welded by a specialist rather than a body-shop bodge, and use our free MOT history check to spot corrosion-prone vehicles before purchase.