Do you know where the supply to the garage comes from – is it via an MCB (fuse) from one of your consumer units?
Does your garage have a consumer unit/fuse box?
If your garage electric supply is 10mm2 cable there will be no problem having a charger fitted in the garage from the electrical supply feeding the garage . Even if your garage supplied by 6mm2 cable, it will be possible. However up untill fairly recently, most garages have the electric supplied by 4mm or 2.5mm2, which wouldn’t be suitable without severely derating the charger, making having one not worthwhile.
The extension consumer unit needs to have 25mm2 tails suppling electric to it, to really make it suitable for adding a charger. Ohme will not fit a charger if the tails have a smaller cross sectional area than 25mm2. Equally if the DNO needs to upgrade your cut-out fuse to 80 or 100A, they like to see 25mm2 tails and could refuse (not as they go poking around anyware other than the meter box), not to mention it’s also best practice.
So you need to know (ask the village electrician) 15min work:
1 – What size cable is feeding the garage supply
2 – What size tails are feeding the extension consumer unit (simply requires removing the lid)
If the cable feeding your garage is suitable, thats great you can have a standard charger install by Motability. (Uunlikely option)
If the tails feeing the extension consumer unit are 25mm2, again you can have a standard charger install by Motability, by having the charger fitted on the outside wall, the consumer unit is on. You can spec an 8m cable and simply run it over to the garage.
Anything more required, would trigger a non standard install which would be way over charged and would be worth getting your own quote, where it “may” be possible to have a charger fitted even if the tails that supply to the extension consumer unit are 16mm, depending on design characteristics.
If non of those options are possible (sit down for this), the best and my prefered option would be from the meter box to (a) a charger fitted on the extension wall (b) a new 10mm2 supply going to the garage, allowing the charger to be installed in the garage and to power the garage/shed sockets/lights.
Some neat trunking in the porch if the cable can’t be hidden, comming out at skirting level and clipped at low level around the external building fabric using D Line clips (invisable) compared to cleats and then a trench dug and into the garage, where a new supply and consumer unit will be fitted or to a charger on the outside wall of the extension the consumer unit is on and to finish off I wuld paint the cable white lol.