I did a garage conversion at my last house. I basically had to key the bricks into the existing walls and then brick around to make a window that took up most the top half of the hole left by the garage door.
Breeze block outer skin, thermal breeze block inner from memory.
Got the plasterer who did the walls inside to render the outside wall to match the rest of the house. Easy.
Plasters only comment was that for one of the blocks I had the 'cut' end facing the window frame instead of into the wall. I had spotted that when I was three courses higher but couldn't face taking it all down again when it would be hidden anyway.
Contemplating doing the Kitchen extension at the new place.