Planning may not be relevant depending on where the office is built and the proposed height. Normally under 2.5 metres high is allowed as long as you don't cover half the garden. 15 square metres is about building regulations.
www.planningportal.co.uk/info/200130/common_projects/56/working_from_home/3
I'm thinking of doing a 4.5 x 4.5m one at 2.5m high. This size would need building regs. However that isn't a bad thing because they determine the fire resistance, structural integrity, electrical install and insulation. I'd also like a loo and shower in there for use as a gym so the plumbing and probably electric shower would add to complexities
What is the size of your garden? I have a long, narrow garden so 4.5 m allows construction within the boundaries and I can leave the fence alone. I don't want the wall of the garden office to form the boundary so next door kick a ball against it or attach to it in future!
I'd go for a pitched roof if possible because they last longer than flat, but it is a balance of headroom and building size in small gardens due to the 2.5m height restriction .
Mine would be south facing so I'm planning on planting a tree for shade! A flat roofed building will cook in the summer in full sun. I like the idea of a canopy to shade the glass in summer and have somewhere to sit if it rains! Also maybe hang a hammock across supporting posts...
Also you need to plan your windows. I can only have front (south) facing windows. Will probably go for bifolds or french doors with windows either side to maximise glass for light and openings to manage heat. possibly a skylight for the back of the room.
I want to insulate to building regs so it is usable in winter. Due to distance from house will need electric heating rather than gas. Possibly underfloor heating? Supplemented with an electric heater. A log burner seems too much hassle, but would be nice, but I can see them being banned completely in the future in London.
All this electric will need a decent cable from house to office - so talk to an electrification early in your design!
I'm even wondering whether to go for a brick & block structure. it is more money, but then pretty much no maintenance. I hate treating my shed and fence.
So a number of things to think about. Start with the size of your garden and what you can fit in there. That will drive other choices.
I was looking at this log cabin as a base - but would need to add so much (electrics, insulation etc). but the dimensions and style are close to what I want. www.tuin.co.uk/Ipswich-Log-Cabin.html