You need to verify that your earthing and bonding are up to snuff and that the extra load will not be an issue.
You will need to inspect and test the existing circuit and verify that it is suitable for the addition you propose.
If the existing socket is just a spur you will almost certainly need to install a fused spur unit.
You will need to ensure that these sockets are RCD-protected - you can incorporate one into your fused spur if the circuit does not already have RCD protection.
If your garage is detached from the house you will need to notify building control or employ a registered electrician.
I was under the impression that a detached garage was not in a special location/installation, and as long as the work does not include outside work you do not need to notify Building control. If all your going to do is extend the circuit, then I don't see a problem. I believe it mentions this on page 8 of the Approved Document. (Link below)
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