Here is ours. It is 12 ft x 8 ft (3.7m x 2.4m). Nothing fancy but it is double skinned and insulated with double glazing for winter use and has full electrics.
We originally built it without any purpose in mind, and before Covid used it as an art room and had a big 55” TV in there for movie nights and watching Premier League matches with the kids. Outside of summer the wicker chairs just slot back in.
Since Covid, it has paid for itself as a dual office, as both the missus and I work from home.
STRONGLY recommend electrics. You can't do anything without it.
edit - since we had ours done last year, I give you an idea of cost. £7k including erection for the building (unpainted), £700 for a concrete slab, £1k for electrics. We needed a new RCD fuse board, so you can almost halve the electrical cost this if you don't. Budget for around £8-9k if you don't want to lift a finger of your own.