That's just ridiculous. How is 5x6m "not that large". Its too large for an office where all you're going to do is sit at a desk and work.
Back on Earth and for half the price you can get a 10m2 properly insulated and clad garden office. One example quote I got.
£7500 Includes:
Wooden base with 4x2 joists, insulation in between joists and EPDM under, 18mm OSB on top and laminate flooring.
Decking 500 wide along the front of the building.
Walls 4x2 joists with insulation in between.
11mm external OSB.
DPM membrane all around the building and finished with cladding.
Internal walls and ceiling plasterboarding painted finished (We can also do cladding optionally).
Roof with 6x2 joists insulation in between, 18mm OSB on top, rubber cover on top of OSB.
24 electric points: internal spotlights, external spotlights, sockets, switches and a fuse box all fitted/finished (electric connection to the property main fuse box will cost extra £400)
One 1500x2000 french door.
Concrete base £90 per SQM.