A knocked mirror is one of the more stressful car park discoveries - but before assuming the worst, it's worth spending 30 seconds working out exactly what's broken. The repair is very different depending on what took the damage.
Step 1 - Work out what's actually damaged
Stand back and look at the mirror properly. There are three distinct scenarios:
The glass is cracked or missing, but the housing is intact. This is the best-case outcome. The plastic casing is fine, the motor is fine, only the glass needs replacing. That's a clip-on fit - two minutes and done. See the wing mirror glass fitting guide.
The housing cap is cracked, chipped or broken, but the mirror still folds and moves normally. The cap is a separate plastic part that can be replaced on its own. The motor housing inside is fine. Replacement caps are available in the mirror covers collection by make and model.
The whole mirror is hanging off or snapped clean away. The mounting bracket or the mirror arm has broken. This usually needs the full mirror assembly replacing, and depending on the car may require removing the door panel to access the mounting bolts. This one is more involved - either a garage job or a careful DIY project if you're comfortable with a bit of dismantling.
If it's just the glass
The glass clips onto the motor housing inside the mirror casing. Even if the glass is completely gone - knocked out entirely - the housing itself is usually undamaged. Order the correct clip-on replacement glass for your make, model, year and side from the wing mirrors collection, and it clips straight on. Driver's side typically takes wide angle glass, passenger's side takes convex.
If the mirror is heated, the heated replacement glass has two small connector pins on the backing plate that plug into the motor housing. Non-heated glass clips on without any connections.
If the housing cap is cracked
The cap is the coloured plastic shell on the outside of the mirror. It protects the motor and glass from the elements but isn't structural. On most cars it either clips on or is held by one or two small screws accessible from inside the housing.
A cracked cap doesn't affect the mirror's function - it'll still fold, still adjust, the glass is still intact. But it looks bad, and a crack that allows water ingress can eventually damage the motor. Worth replacing sooner rather than later.
Replacement caps are in the mirror covers collection. Most come unpainted - they can be used as-is if you don't mind a colour mismatch, or taken to a body shop for painting to match.
Can I drive with a knocked-off mirror?
It depends on which mirror and how much is missing. UK law requires a minimum of two working mirrors. If the passenger side mirror is damaged and your interior mirror and driver's side mirror are both intact and unobstructed, you're within the legal minimum for a short journey. The driver's side and interior mirror are the legally required pair.
That said, a missing passenger mirror is an MOT failure, a noticeable blind spot, and technically a defect you're expected to fix promptly. It shouldn't be a long-term situation.
If the driver's side mirror is damaged or missing, don't drive. That's the side that matters most for overtaking, junction exits and lane changes. Sort it before you use the car.
Does a knocked mirror need to be reported to insurance?
If another vehicle hit it and drove off, it depends on your policy whether you'd want to claim - the excess may well be more than the cost of the repair. If you're in a car park and the culprit left a note, that's a different situation. For a small glass replacement at £10-40, most people find it easier to fix and forget rather than involve insurance.
Common questions
My mirror glass is gone but the frame is fine - is that common?
Yes, fairly common. The glass is designed to be the sacrificial part - the backing plate clip is meant to release before the housing cracks. A glancing blow or a wing mirror brush in a narrow gap often pops the glass off cleanly while leaving the housing intact. Which is why clip-on replacement glass exists.
The mirror folds in but won't fold back out - is that related?
Possibly. A hard knock can damage the fold mechanism or displace the housing from its pivot. If the mirror was working normally before the incident and now won't extend, the fold motor or the pivot joint may have taken damage. That's worth checking before assuming it's just a glass issue.
My replacement glass keeps falling off - what's wrong?
The clip plate on the new glass isn't engaging properly with the motor housing. This usually means either the wrong glass was ordered (different clip profile), the housing has a broken clip point, or the glass isn't being pressed in far enough. Press firmly around the full perimeter, not just the centre. If a clip point on the housing is broken, the backing plate won't lock regardless of how hard you press - the housing would need replacing in that case.