I would look at changing the supply cable for the garage, do some cable calculations to see what you need in there taking into account diversity etc. When you do the EIC for it attach a copy of your calcs so when the assessor comes round you can show him what you have done and why etc, even reference BS71671, OSG etc. This will show him/her you have used the books and designed you installation to the regulations, may sound a little excessive but if you prepare well for this you should have no problems getting a tick in the box.