Can anyone provide advice on building a garden office in SE23?

Whilst the short jaunt to Canary Wharf via the Overground is never too much of a chore from our lovely home of Forest Hill, there are days when I would be much more productive avoiding the commute and "working from home". The problem is home is a chaotic, messy, space constrained zone that is atypical of modern family life. It has led me to conclude that the 100ft garden requires a 'man escape' zone and dedicated work space

I wonder if anyone on the forum has embarked on such a venture? I'm keen to know about any challenges, I'm most concerned about building regulations and planning permission, as I believe we are in a conservation area (Devonshire Road). Similarly, I'm keen to be sympathetic to eco and environmental demands, and neighbours alike, this is not intended as a "garden grab" for space, more an outbuilding that compliments the natural surrounds.

Any advice or pointers from the collective wisdom of the SE23'ers would be greatly appreciated