The answer to your selected question is shown below.
Yes, this is perfectly possible but with some attention to the specification of the roller door used.
Nearly all the aluminium double skinned electric roller garage doors we manufacture and sell are able to be installed externally. This is a common practice when the headroom inside a garage is limited or there are other obstructions inside the garage, but also to maximise the length of the garage if your car needs every millimetre to fit inside lengthways and close the door.
The roller door fitted outside will always require a full hood cover to protect the mechanism, curtain barrel and motor from the elements, but also to provide safety and help prevent fingers being trapped in a roller door opening upwards.
External roller doors are nearly always aluminium, foam filled models, with smaller roll sizes when open fully, enabling smaller and more discreet aluminium hood covers to be fitted. Most hood covers are about the 300mm by 300mm size, preferably with a chamfered front edge to make a softer visual aesthetic.
Please be aware that for an external installation you will always be viewing the 'inside' face of the individual aluminium slats used for the curtain, and this is the concave side and not the slightly convex side usually showing for any roller door manufactured in this way. Be aware too some finishes and colours may be only relevant on the convex side (the outside) and the inside face may be a different colour altogether. We generally ask for details of where the roller door is being installed to check on this for you.
When you install a roller garage door outside the opening care must be taken to ensure you order optional lockable cover plates for any manual override system provided, as these will be otherwise exposed to anyone outside, and also ensure you realise the electric motor side specified in the ordering process may be taken as the side looking from outside and not inside, It is usually taken from looking at the curtain roll and hood cover, regardless of where it is fitted.
The ranges of steel 'continuous' curtain non insulated roller garage doors also on the Uk market are not suitable for external installation as they have guides and bracket supports not suitable for external use as well as having a much larger size of roll to deal with with no obvious way of securing a hood cover to anything around the roller curtain.
Hood covers are only sold for aluminium roller doors and are nearly always made of aluminium as well with various powder coated finishes to match or contrast the door curtain.
it is possible to use either a Compact roller door or even one of the latest 'Qompact' roller doors which have box sizes of 205mm and 165mm respectively, far smaller than the standard door hood cover and roll sizes.
The image below shows an externally fitted roller garage door with a hood.