It is possible that you might need planning permission is you are going to use your garage for a different purpose. This change in purpose might necessitate you having different plans for your garage, and getting the purpose of it changed on the deeds. However, this is different to actual planning permission as you will not actually be building onto your garage, just changing its structure.
However, you do not want to risk having a garage that does not meet with regulations. It is always best to check with your solicitor to make sure you are doing everything above board and to the right plans. If you are going to be extending your garage at all or changing its footprint, even if it is just to join it up to your house, then you will need to ensure you have the right planning permission in place before any building work goes ahead.