Worked in mine for eight years. [ 4 m x 3 m ]
Make sure the insulation is good. This one actually needs no heating at all.
The lighting is sufficient to warm it.
Special things in the design were three sides totally solid, but one end is floor to ceiling glass windows and double doors. Gives great view to the garden and gets the sun coming round all day.
No kitchen or toilet, they're not necessary, and it's nice to take a break back in the house.
Velux windows in both sides of the roof, as it can get too warm...also allows natural light.
Make sure you have enough electric points.
Make sure the floor is really strong, as the loading from furniture, shelving, machines and accumulated files and stuff does add up.
Advantage of being very inexpensive and I love the commute.
Disadvantage is popping in for a few minutes in the evenings and at weekends, which can become hours! and our cat has a habit of finding a warm corner or shelf, and gets locked in.