Single garage conversion ideas run into one constraint that a double never does: width. A British single garage is sized for a car and its doors, and once an insulated build up goes on both walls the finished room is narrow enough that only some uses work in it. The ideas worth having are the ones that suit a long narrow room rather than the ones that fight it.
Work with the proportion instead of against it
A long narrow room is excellent as a home office with the desk at the window end, as a utility with everything on one wall, as a snug with a sofa down one side, or as a playroom. It is poor as a dining room, awkward as a double bedroom, and it will never be the open plan space the photographs show, because those are doubles.
Put everything on one wall
Narrow rooms work when the traffic route is clear. Running storage, worktop or a desk along one side and leaving the other clear gives a usable room; splitting furniture between both sides gives a corridor with obstacles. That single decision does more for the room than any finish.
Use the door opening for light, not for a door
Filling the opening with a large window, or a window and a low panel, puts daylight at the short end where a narrow room needs it most. A door there is usually a worse use of the only wall that can take glass, unless the room genuinely needs its own entrance.
Consider losing the width to gain the warmth, or the other way round
Insulating internally is cheaper and takes width. Insulating externally keeps the width and changes the elevation, which is often acceptable at the side or rear and rarely at the front. On a single garage this is a real decision rather than a technicality, because a hundred millimetres each side is the difference between two layouts.
Questions people ask about single garage conversion ideas
What are the usual dimensions of a British single garage?
They vary and most are built to take one car with the doors closed, which is not much more than the car. Rather than working from a typical figure, measure your own internally and subtract the wall build up, because that number is the room.
Can a single garage be a bedroom?
It can where the width works and where an escape window and fire separation are provided. Whether it should be is a question about the width and the light rather than about the regulations.
Should I knock through into the house?
If the garage adjoins a room you want to extend, opening up gives a much better result than a separate room, and it turns the job into structural work with a beam and a calculation. It also removes the fire separation question by making the two one space, which has its own requirements.