Couple of things as I did this (Raised 1 foot)
1. You must buy a new garage door. It is theoretically possible to add another section, but they are not built in nice 1 foot increments. They are more like 19" or 21". Contact your garage door people for full details.
2. You must have room in the wall above to raise the header up. For example, the header supporting the load is probably somewhere between 12" and 18" tall. The top of that has to be able to go upwards the required dimension and still stay under the double top plate in the framing.
3. You have to have some way of supporting the building load while cutting out the header. Most importantly if its on a sidewall with roof trusses bearing down on it. Less temporary structure required for an endwall door.
Its definitely not recommended for average DIYer. I would get construction pros involved. I'm going to guess $2k to $5k to do this job depending on how complex the garage is and what's over top, if the sidewall height does actually exist to do it.
Also, it needs to be stated that if you have an 8 foot wide trailer, a 9 foot door is going to be like threading the eye of a needle with it. A 10 foot wide door would be my minimum, and even that is going to require very careful work with the tow vehicle and a spotter. Putting a trailer into a dark unlighted space in bright daylight outside is akin to shooting in the dark....
