I've decided I don't want to move but would like a slightly bigger 3rd bedroom and a spare room and we can't extend out the back so would make sense to go forwards as the 3rd bedroom looks on to the garage anyway.
My only worry is if we would be able to as its a terraced house so our garage shares a wall with our neighbour if that makes sense? Very similar to this house! The 3rd bedroom is where that small window is and we would want the whole width ideally!
I'm not an expert, but I would have a good look around your area to see if anyone else has done the same to their homes. You can often look up planning applications online with your local council. I know that some applications aren't look on favourably if they make the house look more 'terraced' ie, they have not gap between the houses. You house might rise in value with the extra rooms, but your neighbours might drop due to the terrace effect. I would also suspect many garages aren't built to support an upper storey so you might have to rebuild the whole side, which would be difficult if your neighbour shares the garage wall.
Most local planning authorities would be looking to make sure that the extension would not have a significant impact on neighbours or on the appearance of the terrace.
With regards to neighbours, building up above the garage may have an impact on outlook and light to the front facing windows next door. Depends on where the sun tracks and height/forward projection.
With regards to the appearance of the terrace, street scene etc, if this isn't a common type of extension in the terrace or nearby, then only one garage being built above would stick out and may appear visually jarring and out of character.
Most local authorities will provide pre-application planning advice. Though it's usually subject to a fee nowadays.
All the houses are the same and no one has extended out the front, some have out the back but as we've just spent a significant amount of money on bifold stage width of the house and our garden isn't big enough to loose any really
Another thought you would need to investigate if the footings would be suitable support another story. A similar situation with my parent's the footings were not suitable and it was cheapest to pull single story down and rebuild.