Insurance on a modified car

[speedking31]

How does insurance work for a modified car?
All numbers for demonstration purposes only.

Say you have a car which in standard form has a market value of £5k.
You spend £10k on modifications, but accept that you never get the whole value back, so say the car is ‘worth’ £10k to you. All modifications declared and increased premium being paid.
If you have an accident causing £6k of damage. Is the car written off? How is market value determined for a car with unique modifications? What you think adds value, e.g. bigger turbo, may make the car harder to sell.
If the insurer is to restore you to pre-accident condition, a standard car would have to have the £10k spent on it again, i.e. £15k value. Does it make any difference if the modifications are not part of the damage, e.g. extensive engine mods but involved in a rear end shunt?
All the above assuming it is not an “agreed value” policy.
Can someone explain the principles.

[Troubleatmill]

It depends on what the policy says.

Some policies may cover you in the event of a prang - but you get nothing back apart from OEM stuff.
Other policies may replace modified bits like for like.

YOU really need to read exactly what YOUR policy covers YOU for.

[speedking31]

In each of your examples, if the modified parts themselves are not damaged then you just get the market value of a standard car?

[e21Mark]

I have an extensively modified e21 BMW that is valued at £15k, although I doubt it would sell for that amount. My premium is about £290 I think? It cost more than the insur value to build but I had to be realistic.

[kambites]

This is why agreed valuation policies exist.